President Volodymyr Zelenskyy accused Russia on Tuesday of playing for time to be able to continue the war in Ukraine and its occupation of Ukrainian territory.

Speaking a day after President Donald Trump discussed the war with Russian President Vladimir Putin, Zelenskyy said the conflict must be ended through negotiations but that there must be clear and realistic proposals on the table.

Zelenskyy, who said he had spoken to Finnish President Alexander Stubb about the peace process, also called for more international sanctions to increase pressure on Russia.

"It is obvious that Russia is trying to buy time to continue the war and occupation," Zelenskyy wrote on the Telegram messaging app.

"We are working with partners to put pressure on the Russians to behave differently. Sanctions matter, and I am grateful to everyone who makes them more tangible for the perpetrators of the war."

Trump said Russia and Ukraine would immediately start negotiations on a ceasefire, but the Kremlin said the process would take time and the U.S. president indicated he was not ready to join Europe with new sanctions to pressure Moscow.

Trump also said he had relayed the plan to Zelenskyy as well as the leaders of the European Union, France, Italy, Germany, and Finland in a group call.

"We have no doubt that the war must end at the negotiating table," Zelenskyy said on Telegram.

"There must be clear and realistic proposals on the table. Ukraine is ready for any effective negotiation formats. And if Russia continues to put forward unrealistic conditions and undermine possible results, there must be tough consequences."

Putin, whose forces control a fifth of Ukraine and are advancing, has stood firm on his conditions for ending the war, including the withdrawal of Ukrainian troops from four Ukrainian regions Russia claims.