(CNN) Russian Foreign Minister Sergey Lavrov has questioned the framing of NATO as a "defensive" alliance, complaining that the bloc's "line of defense" continues to move east towards Russia, as fears mount that the Kremlin is planning an invasion of Ukraine.
"Let them console themselves with saying that it is 'defensive.' It does not make things easier for us," Lavrov told Russia's Channel One evening show on Sunday. "The line of defense has already come close to us."
"It turns out each time that the line they are supposed to defend is moving further east. Now, it has already come close to Ukraine," Russia's top diplomat added.
His comments came amid heightened tensions on the Russia-Ukraine border. British Foreign Secretary Liz Truss said Sunday it is "highly likely" that Russia "will invade Ukraine."
"As we saw in Chechnya, as we saw in Afghanistan, the Ukrainians will fight -- that is absolutely clear," she told the BBC.
Lavrov said an eventual accession of Ukraine to NATO would "really undermine relations with the Russian Federation."
"It will be a gross violation of obligations taken by the presidents of the US and other member states of the alliance," the Russian Foreign Minister said.
Lavrov said his office would be sending requests for clarification to his counterparts in NATO and the OSCE.
"[We are sending] a pressing request to explain how they intend to fulfil the commitment not to strengthen their security at the expense of security of the others," he said. "If they don't plan this, then they should explain why. This will be the key question in determining our further proposals that we will report to Russian President Vladimir Putin."
Lavrov also claimed Ukraine's accession to the North Atlantic alliance made little sense, from a strategic standpoint.
"It is clear to everyone that Ukraine is not ready and it won't make any contribution whatsoever to the strengthening of NATO's security," he said.
Earlier on Sunday, Secretary-General Jens Stoltenberg told the BBC the group has no plans to deploy combat troops to Ukraine as the country is not part of the alliance.
"We have no plans to deploy NATO combat troops to Ukraine...Ukraine is not a NATO ally, so the 100% security guarantees that the attack on one ally will trigger a response from the whole alliance, applies to NATO allies not to a close and valuable partner," Stoltenberg said.