Ukraine
The PM's spokesman also condemned Russia's "barbaric" overnight attacks.
The US claimed the Russian president had agreed to a ceasefire on "energy and infrastructure".
The Russian president has agreed to halt strikes on energy plants for 30 days after a call with Donald Trump.
The Russian president seemed pre-occupied by discussions with oligarchs.
"Well, I was being a little bit sarcastic when I said that," the president said.
The PM has promised more than 30 countries could join the informal alliance to back Ukraine.
No.10 said "a significant number" have agreed to send troops to Ukraine in the event of a peace deal.
Troops deployed to the frontline should not operate as peacekeepers, according to Chatham House's Keir Giles.
"We want to see if we can bring that war to an end.”
"If it looks like he is using cuts to welfare to pay for war I'm not sure that's going to be popular at all"