(CNN) For the first time, the NATO Response Force has been activated as a defensive measure in response to Russia's ongoing invasion of Ukraine.
NATO Supreme Allied Commander Gen. Tod Wolters activated the multinational force consisting of land, air, sea and special operations forces from the allies that can deploy quickly in support of the NATO alliance.
The activation of the response troops does not mean that any US or NATO troops will go into Ukraine, which is not a member. US President Joe Biden has been clear that US troops are deploying to eastern Europe to help bolster NATO countries nervous about Russia's aggressive actions, and they will not be fighting in Ukraine.
"This is an historic moment and the very first time the Alliance has employed these high readiness forces in a deterrence and defense role. They represent a flexible, combat credible force that can be employed in multiple ways and we are utilizing fully their inherent agility," Wolters said in a statement "These deterrence measures are prudent and enhance our speed, responsiveness and capability to shield and protect the one billion citizens we swore to protect."
The force has not yet deployed but is on standby. It's not clear how many troops the response force will contain.
The decision to activate the response force was announced following a call Friday morning with NATO leaders. In a statement on the call, Biden said NATO was staying unified to stand up to Putin's threats to international peace and security.
"We were joined today by our close partners Sweden, Finland, and the European Union," the statement reads. "President Putin has failed in his goal of dividing the West. NATO is as united and resolute as it's ever been, and NATO will maintain its Open Door to those European states who share our values and who one day may seek to join our Alliance."
NATO Secretary-General Jens Stoltenberg said Friday that the alliance still needed to do more to put pressure on the Kremlin to bring an end to military aggression in Ukraine.
"We must stand ready to do more, even if it means we have to pay the price, because we are in this for the long haul," he said. "We have to take this seriously, and that's exactly why we are now deploying the NATO response force for the first time in a collective defense context," he said.
The entire NATO Response Force is 40,000 troops but Stoltenberg did not say how many would be activated, telling reporters "we are deployment elements" of the force.
In a message released Friday morning, Zelensky thanked NATO for its support but urged the alliance to do more, lamenting the fact that Ukraine was left to fend for itself.
"We are alone in defending our country. Who is ready to defend us together with us? Honestly, I don't see any," Zelensky said. "Who is ready to give Ukraine a guarantee of joining NATO? Honestly, everyone is afraid."
The White House said that Biden spoke by phone on Friday with Zelensky, who posted a video showing he's still in Kyiv. Biden's statement said the US President "conveyed ongoing economic, humanitarian, and security support being provided by the United States."
Pentagon press secretary John Kirby told reporters Friday that the US is "going to provide additional security assistance" to Ukraine, though how it will be done, he noted, was "still being worked out."