French President Emmanuel Macron, German Chancellor Olaf Scholz and US President Joe Biden agreed that “this step will not go unanswered”, the chancellery said in a statement published following their conversation.
The three Western allies also vowed not to let up in their commitment to the territorial integrity and sovereignty of Ukraine.
Praising the restraint shown by Ukrainian President Volodymyr Zelensky over the latest developments, they added that they will “do everything in their powers to prevent a further escalation of the situation”.
Late on Monday night, Putin is believed to have ordered troops into two Russian-controlled regions of southeast Ukraine, Donetsk and Luhansk, after declaring these seperatist areas “independent and sovereign” states.
In a televised speech that lasted around an hour, the Russian leader said his troops would be sent into the regions “on peacekeeping duties”.
Responding to the sudden news, German Foreign Minister Annalena Baerbock said that Moscow was breaking the Minsk peace agreements that it signed in 2014.
“With its decision, Russia is breaking all its promises to the world community,” she said.
Ahead of Moscow’s signing of the new declaration, Olaf Scholz had used a phonecall with Vladimir Putin to stress that “such a step would be a gross contradiction of the Minsk agreement for a peaceful settlement of the conflict in east Ukraine and a unilateral breach of these deals from the Russian side”.
Scholz, whose country currently holds the presidency of the G7, also urged Putin to “immediately deescalate and withdraw the massed forces from the borders to Ukraine”.
“He stressed that it was now particularly important to observe the ceasefire in eastern Ukraine and to set signs of detente,” according to a statement issued by the chancellery.
“Russia has a particular responsibility here,” he added.
With troops having allegedly crossed the border into Ukraine, there are fears that Putin is paving the way for a wider invasion.
Western leaders have placed initial sanctions on Russia following the breach.