Donald Trump has warned he will impose high tariffs and further sanctions on Russia if Vladimir Putin fails to end the war in Ukraine.

Writing on his social media platform Truth Social, he said that by pushing to settle the war he was doing Russia and its president a "very big favour".

Trump had previously said he would negotiate a settlement to Russia's full-scale invasion launched in February 2022, in a single day.

Russia has not yet responded to the remarks, but senior officials have said in recent days that there is a small window of opportunity for Moscow to deal with the new US administration.

Putin has said repeatedly that he is prepared to negotiate an end to the war, which first began in 2014, but that Ukraine would have to accept the reality of Russian territorial gains, which are currently about 20% of its land. He also refuses to allow Ukraine to join Nato, the military alliance of Western countries.

Kyiv does not want to give up its territory, although President Volodymyr Zelensky has conceded he may have to cede some currently occupied land temporarily.

On Tuesday, Trump told a news conference he would be talking to Putin "very soon" and it "sounds likely" that he would apply more sanctions if the Russian leader did not come to the table.

But in his Truth Social post the next day, he went further: "I'm going to do Russia, whose Economy is failing, and President Putin, a very big FAVOR," he wrote.

"Settle now, and STOP this ridiculous War! IT'S ONLY GOING TO GET WORSE. If we don't make a 'deal', and soon, I have no other choice but to put high levels of Taxes, Tariffs, and Sanctions on anything being sold by Russia to the United States, and various other participating countries."

Continuing, he wrote: "Let's get this war, which never would have started if I were President, over with! We can do it the easy way, or the hard way - and the easy way is always better. It's time to 'MAKE A DEAL'."

Trump's former special representative for Ukraine, Kurt Volker, said Trump's threat of more serious sanctions on Russia "gives a signal to Vladimir Putin this is going to get worse, not better". Speaking to BBC Radio 4's Today programme, he added: "We should incentivise Putin to say, 'OK, it's time actually to have a ceasefire.'"

Russia's deputy UN ambassador Dmitry Polyanskiy earlier told Reuters news agency that the Kremlin would need to know what Trump wanted in a deal to stop the war before the country moves forward.

Meanwhile, Ukrainian President Volodymyr Zelensky told the World Economic Forum on Tuesday that at least 200,000 peacekeepers would be needed under any agreement.

And he told Bloomberg that any peacekeeping force for his country would have to include US troops to pose a realistic deterrent to Russia.

"It can't be without the United States... Even if some European friends think it can be, no, it will not be," he said, adding that no-one else would risk such a move without the US.

While Ukraine's leaders might appreciate this tougher-talking Trump - they have always said Putin only understands strength - the initial reaction in Kyiv to the US president's comments suggest that it is actions people are waiting for, not words.

Trump has not specified where more economic penalties might be aimed, or when. Russian imports to the US have plummeted since 2022 and there are all sorts of heavy restrictions already in place.

Currently, the main Russian exports to the US are phosphate-based fertilisers and platinum.

Speaking to the BBC, Volker said the Russian economy could take "substantial" damage if Trump chose to preserve or strengthen the toughest US sanctions so far, which he said were only levied as Trump's predecessor Joe Biden left office. "Russia really didn't feel as much pressure as they could," he commented.

On social media, there was a generally scathing response to Trump's comments from Ukrainians. Many suggested that more sanctions would be a weak reply to Russian aggression. But the biggest question for most is what Putin is actually open to discussing with Ukraine at any peace talks.

In Moscow meanwhile, some people are seeing signs that the Kremlin may be readying Russians to accept less than the "victory" once envisaged, which included tanks rolling all the way west to the southern Ukrainian port city of Odesa.

TV editor Margarita Simonyan, who is stridently pro-Putin, has begun talking of "realistic" conditions for ending the war, which she suggests could include halting the fighting along the current frontline.

That would mean the four Ukrainian regions that Putin illegally pronounced as Russian territory more than two years ago, including Zaporizhzhia, still being partially controlled by Kyiv.

Russian hardliners, the so-called "Z" bloggers, are furious at such "defeatism".

In his BBC interview on Thursday, Trump's former envoy Volker said he was "sceptical that there is going to be a deal per se", adding that the first priority of the US could be to stop the fighting and then deter more attacks by Putin.

In Wednesday's social media post, Trump also couched his threat of tariffs and tighter sanctions in words of "love" for the Russian people and highlighted his respect for Soviet losses in World War Two – a near-sacred topic for Putin.

But Trump massively overestimated the numbers and appeared to think the USSR was Russia alone. In reality, millions of Ukrainians and other Soviet citizens also lost their lives.

That said, the man who previously said he could "understand" Russia's concerns about Ukraine joining Nato - which for Kyiv is tantamount to saying Putin was provoked - does seem to be shifting his tone.

Trump's position matters. But after 11 years of war with Russia and a history of poor peace deals, Ukrainians are not inclined to be too hopeful.