KYIV, May 20 - Volodymyr Zelenskyy has accused Russia of "trying to buy time" to continue its war in Ukraine, a day after Donald Trump said progress towards a ceasefire had been made in a call with Vladimir Putin.

"If Russia continues to put forward unrealistic conditions and undermine progress, there must be tough consequences," Ukraine's president wrote on social media, adding that Kyiv was ready to negotiate, BBC reported.

Following separate calls with Zelenskyy and Putin on Monday, Trump said truce talks between Russia and Ukraine would start "immediately".

Putin said he was ready to work on a "memorandum on a possible future peace agreement", but did not address calls for a 30-day pause in fighting.

On Tuesday, Russian Foreign Ministry spokeswoman Maria Zakharova said it was up to Kyiv to "make decisions" and respond to a proposed peace memorandum.

However, the Kremlin downplayed suggestions that negotiations were close, with Russian state news agencies citing spokesperson Dmitry Peskov as saying "there are no deadlines and there cannot be any".

Meanwhile, Zelenskyy launched a fresh round of diplomacy, speaking to Western allies in a bid to shore up support.

Following a phone call with the Finnish president, Zelenskyy wrote on social media that Ukraine was working with partners to ensure pressure on Moscow "forces the Russians to change their behavior".

It came as the European Union and UK announced that they had adopted new rounds of sanctions against Russia.

The EU said it was blacklisting nearly 200 more oil tankers in Russia's "shadow fleet", and warned of a "tougher response" if Moscow did not agree to a truce in Ukraine.

The UK placed sanctions 18 more tankers, as part of its package targeting Russian military suppliers, energy exports and financial institutions.

It followed a record drone assault in Ukraine over the weekend, which Kyiv officials described as the largest since the start of the full-scale invasion.

On Monday, Trump indicated he would not join in any new sanctions on Russia, telling reporters that they could hinder recent progress.

It came as heavy fighting continued along the front line, with Ukraine reporting 177 clashes and claiming over 1,000 Russian casualties in 24 hours.

Speaking ahead of a meeting of EU ministers on Tuesday, German Defence Minister Boris Pistorius said Russia's continued attacks "speak louder than the lip service we have heard for so long".

"Putin is clearly playing for time. Unfortunately, we have to say he is not really interested in peace," he said.

Putin has so far rebuffed a joint US-Ukrainian proposal for a 30-day ceasefire and last week rejected Zelenskyy's invitation to meet for talks in Istanbul. Trump had offered to attend the talks if Putin would be there, but the Russian leader declined.

The Kremlin has previously announced temporary ceasefires, including from 8-11 Mayto coincide with Russia's Second World War victory celebrations. Kyiv refused to join, citing mistrust and demanding an immediate, sustained cessation of hostilities.

A similar 30-hour truce over Easter saw a brief lull in fighting, though both sides accused each other of hundreds of violations. (bbc/ez)