(CNN) Ukrainian President Volodymyr Zelensky is issuing his most fervent appeal yet for ceasefire talks with Vladimir Putin, but the so-far immovable Russian President is delivering a ruthless response by escalating atrocities against civilians.
Zelensky told CNN's Fareed Zakaria on Sunday he is ready to talk to Putin at any time. His offer came more than three weeks into a war that appears to be entering a new, more deadlocked stage on the battlefield. And while that counts as an extraordinary military success for outgunned Ukraine, it will also leave its cities and people even more vulnerable to brutal Russian bombardments.
"It's a stalemate. But we should note it's a bloody stalemate," ex-CIA Director David Petraeus, a retired general who served in Iraq and Afghanistan, said on CNN's "State of the Union" on Sunday. "Also, arguably, it's a battle of attrition."
Moscow's troops are digging in around Kyiv amid increasing doubts about their capacity to capture the capital. Pitched battles are taking place in the south, where the Russians are targeting Black Sea access in the cities of Mariupol and Odessa, and in the east of the country. Russians have reportedly taken heavy casualties along with the loss of a string of senior officers. And the Kremlin is increasingly resorting to murderous stand-off barrages from missiles and artillery and even, in an alarming escalation, hypersonic weapons after their initial expectations faded of a blitzkrieg that could quickly seize control of Ukraine.
In the latest outrage on Sunday, Russian bombs slammed into an arts school in besieged Mariupol where 400 people had been sheltering from the fighting, according to city officials. Both the government in Kyiv and Mariupol city authorities flatly rejected the terms of a Russian ultimatum that the city surrender by Monday morning. Hundreds of thousands of people are trapped in the city and there are appalling reports of food and water running out and desperate conditions. Some experts fear this is a dark omen of a fate that could befall Kyiv.
The onslaught against civilians appears to be both a deliberate attempt by Moscow to break Ukraine's remarkable morale and resistance -- and to bomb an independent, sovereign nation, which Putin says has no right to exist, to smithereens to crush its dreams of joining the West.
Western governments have responded by pouring anti-tank and anti-aircraft missiles into Ukraine in what is now a proxy war with Russia, which would have seemed an unbelievable prospect only a few weeks ago. Anxiety is still acute that the conflict could spill over into a broader conflagration between the US and Russia -- the world's top nuclear powers.
This is the tense situation that will greet Joe Biden when he makes the most critical trip by a US president to Europe in many years this week, albeit one with limited expectations of a breakthrough in diplomatic efforts to end the war.
Ahead of Biden's mission, some details are now emerging of the potential parameters of negotiations aimed at reaching a ceasefire. Zelensky wants in-person talks with the Russian leader to hammer them out.
"I'm ready for negotiations with him. I was ready for the last two years. And I think that without negotiations we cannot end this war," Zelensky told Zakaria in an exclusive interview.
"I think that we have to use any format, any chance in order to have a possibility of negotiating, possibility of talking to Putin. But if these attempts fail, that would mean that this is a third World War," he said.
President Recep Tayyip Erdogan of Turkey, a member of NATO that also has friendly ties to Moscow, has offered to host both Zelensky and Putin to facilitate negotiations to end the war. But all international diplomatic gambits have so far failed to make even minor progress with Putin.
There is widespread skepticism that the Russian strongman, who has a record of inflicting withering civilian carnage in Syria and Chechnya, has any intention of talking peace yet. While there is a strong argument that Putin has badly miscalculated and visited a strategic and economic disaster on Russia, there is also a scenario in which continuing the war makes sense in his idiosyncratic worldview.
There is no sign yet that the debilitating sanctions and heavy Russian losses have changed his calculations. And destroying whole Ukrainian cities in blunt force, mid-20th-century-style warfare and punishing its people make a stark statement about his intolerance for the expansion of NATO -- a bloc Ukraine once hoped to join. The vast refugee exodus into Western Europe, meanwhile, will pressure the democracies he is dedicated to destabilizing and threaten one of the greatest historic US foreign policy triumphs -- a free, secure and peaceful Europe for nearly 80 years after World War II. And should there be a new Cold War with the West, initiated by Putin's frightening rhetoric about nuclear arms, it would revive a period when Moscow could wield global power and hold rivals hostage with its threats of escalation.
Also weighing against hopes of an immediate breakthrough are the extraordinary national and geopolitical shifts caused by the largest land combat in Europe since the 1940s.
Putin justified the war with a twisted version of history by warning that Ukraine -- a founding member of the Soviet Union -- had no right to its sovereignty or independence and that Ukrainians were culturally and ethnically Russian.
But a sense of Ukrainian nationhood and identification with Europeans have been solidified by the extraordinary show of unity and resistance among civilians and a massive EU effort to support Kyiv and millions of refugees.
As a result, any solutions that involve Ukraine cutting itself off from the West would be tough for Zelensky to sell to his people, especially after so many civilians have been killed in the nation's fight for its life.
Putin -- after effectively wagering the entire sanctions-pummeled Russian economy and his national and international credibility on the invasion -- cannot afford to be seen to have lost. That means he will drive a hard bargain in any ceasefire talks, if he eventually decides he is ready for diplomacy.
In a broader strategic sense, meanwhile, the West needs Putin to be perceived as the loser of the conflict, as it seeks to deter further Russian adventurism -- potentially into NATO states, which could trigger a disastrous confrontation between Moscow and the West that could escalate into a nuclear war.
"Putin must not win this war," Estonia's Prime Minister Kaja Kallas told CNN's Jake Tapper on "State of the Union" on Sunday.
Kallas, who will be at the hastily called NATO summit that Biden will attend this week in Brussels, said that the alliance's strategy should focus on each member strengthening its defense and isolating Russia at "all the political levels."
As well as the broad strategic questions that make a ceasefire an elusive prospect in the short term, the intricate details of the diplomacy underscore the gulf between Russia and Ukraine -- following multiple rounds of inconclusive talks between their delegations.
According to Turkish presidential spokesman Ibrahim Kalin, Putin laid out his requirements for a ceasefire with Ukraine in a call with Erdogan Thursday.
"Mr. Zelensky is ready for (negotiations). Mr. Putin, on the other hand, is of the opinion that positions are not yet close to each other for talks at the leadership level," Kalin said in an interview with the Turkish Hurriyet newspaper published Saturday.
Putin would require Ukraine to declare neutrality, to renounce NATO membership, and to disarm and to declare neutrality based on an Austrian model, which Kyiv has already dismissed as an unrealistic possibility, Kalin said. A European diplomat, however, told CNN last week that the Austrian-style neutrality framework was not dead.
Putin also wants the "denazification" of Ukraine, Kalin added. The Russian leader's comparison of the Kyiv government to Nazis is not only a false charge, it implies that in order for there to be peace, Zelensky and his ministers must step down in favor of a government acceptable to Moscow -- a condition that would be hard for Ukraine to accept and that would effectively end the democracy it is fighting to save.
Zelensky has already made one apparent concession to Russia, saying that Ukraine has to acknowledge it can't join NATO. He has called for a full Russian withdrawal from his country and Western security guarantees to avoid a future incursion -- a framework that might run afoul of Russia's desire to purge Ukraine of Western influence. And the status of pro-Russian separatist areas in eastern Ukraine, including Crimea, previously seized by Moscow, is likely to be a tough question that could take years to solve.
There is also deep skepticism that Russia is sincere in ending the war or that it could be trusted to live up to the terms of any agreement. The working assumption in the West is that Putin will seek to control Ukraine one way or the other in the future, according to diplomats who spoke to CNN's Kylie Atwood and Jennifer Hansler.
"We've seen lies, lies and more lies," one European diplomat said. Another added: "We are just not sure if you can trust the Russians to maintain their word, if they say they would respect this neutrality."