By Dmitry Antonov and Andrew Osborn

MOSCOW (Reuters) -U.S. President Donald Trump and his Russian counterpart Vladimir Putin were still "maybes" for what could be the first direct peace talks between Moscow and Kyiv in years after the Kremlin on Wednesday held off disclosing who would represent Russia.

Putin on Sunday proposed direct negotiations with Ukraine in Istanbul on Thursday "without any preconditions". But he did not say who would be attending from Moscow's side and his spokesman was unable to give further details on the matter on Wednesday.

Trump earlier this week urged Ukraine to attend the talks and Ukrainian President Volodymyr Zelenskiy quickly said he would be there, but only if Putin showed up, setting up a diplomatic standoff as part of an apparent contest to show Trump who wants peace more.

Trump said on Wednesday he himself was still considering whether to attend the talks in Turkey but did not know whether Putin would go, something that Zelenskiy has challenged the Kremlin leader to do "if he's not afraid".

"(Putin) would like me to be there, and that's a possibility... I don't know that he would be there if I'm not there. We're going to find out," Trump told reporters aboard Air Force One en route to Qatar.

Trump wants the two sides to sign up to a 30-day ceasefire in what is Europe's biggest land war since World War Two, and a Russian lawmaker said on Wednesday there could also be discussions about a huge prisoner of war exchange.

Zelenskiy backs an immediate 30-day ceasefire, but Putin has said he first wants to start talks at which the details of such a ceasefire could be discussed.

MORE SANCTIONS ON RUSSIA?

Trump, who is growing increasingly frustrated with both Russia and Ukraine as he tries to push them towards a peace settlement, said he was "always considering" secondary sanctions against Moscow if he thought it was blocking the process.

U.S. officials have spoken about possible financial sanctions as well as potential secondary sanctions on buyers of Russian oil.

A Ukrainian diplomatic source told Reuters on Wednesday that Ukraine's leadership would decide on its next steps for peace talks in Turkey once there was clarity on Putin's participation.

"Everything will depend on whether Putin is scared of coming to Istanbul or not. Based on his response, the Ukrainian leadership will decide on the next steps," the source said,

If Putin agrees to join, it would be the first meeting between the leaders of the two warring countries since December 2019. Direct talks between negotiators from Ukraine and Russia last took place in Istanbul in March 2022, a month after Putin sent tens of thousands of troops into Ukraine.

Unconfirmed Russian and U.S. media reports have said that Russian Foreign Minister Sergei Lavrov and Yuri Ushakov, Putin's foreign policy aide, will be in Istanbul and ready to meet their Ukrainian counterparts.

Asked by reporters in a daily briefing on Wednesday if the Kremlin could reveal the make-up of the Russian delegation, spokesman Dmitry Peskov said: "We will do that when we get an instruction to do so from the president."

"The Russian delegation will be waiting for the Ukrainian delegation in Istanbul on May 15," he added.

Trump has said he will send Secretary of State Marco Rubio and senior envoys Steve Witkoff and Keith Kellogg to the talks, while also offering to attend himself.

(Reporting by Dmitry Antonov in Moscow;Additional reporting by Tom Balmforth and by Doina Chiacu and Susan Heavey in Washington Writing by Andrew Osborn and Lucy PapachristouEditing by Gareth Jones)