Putin on Sunday proposed direct negotiations with Ukraine in Istanbul on Thursday "without any preconditions". Late on Wednesday, the Kremlin said the delegation would include presidential adviser Vladimir Medinsky and Deputy Defence Minister Alexander Fomin.

After the Kremlin's delegation announcement, a US official said Trump would not attend, days after saying he was considering the trip.

While Putin had never confirmed he would attend in person, the absence of the Russian and US presidents lowers the expectations for a major breakthrough in the war that Russia started in February 2022.

Zelenskyy was on his way to Turkey, a Ukrainian official said. Earlier, the Kyiv leader said he would participate in the negotiations only if Putin were there.

Trump wants the two sides to sign up to a 30-day ceasefire to pause 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.

Zelenskyy 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.

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.

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

With AAP.