By Tom Balmforth and Vladimir Soldatkin

ISTANBUL (Reuters) -Russia's Vladimir Putin spurned a challenge to meet face-to-face with Ukrainian President Volodymyr Zelenskiy in Turkey on Thursday, dealing a blow to prospects for a peace breakthrough.

The Russian president dispatched a second-tier team of aides and deputy ministers to take part in talks in Istanbul, while U.S. President Donald Trump, on a tour of the Gulf, undercut the chances of major progress when he said there would be no movement in the absence of a meeting between himself and Putin.

Zelenskiy said Putin's decision not to attend but to send what he called a "decorative" line-up showed the Russian leader was not serious about ending the war.

He said he himself would not go to Istanbul, but would send a team, headed by his defence minister, with a mandate to discuss a ceasefire. It was not clear when the talks would actually begin.

"We can't be running around the world looking for Putin," Zelenskiy said after meeting Turkish President Tayyip Erdogan in Ankara.

"I feel disrespect from Russia. No meeting time, no agenda, no high-level delegation - this is personal disrespect. To Erdogan, to Trump," Zelenskiy told reporters.

Zelenskiy backs an immediate, unconditional 30-day ceasefire but Putin has said he first wants to start talks at which the details of such a truce could be discussed. More than three years after its full-scale invasion, Russia has the advantage on the battlefield and says Ukraine could use a pause in the war to call up extra troops and acquire more Western weapons.

Both Trump and Putin have said for months they are keen to meet each other, but no date has been set. Trump, after piling heavy pressure on Ukraine and clashing with Zelenskiy in the Oval Office in February, has lately expressed growing impatience that Putin may be "tapping me along".

"Nothing's going to happen until Putin and I get together," Trump told reporters aboard Air Force One.

DIPLOMATIC CONFUSION

The diplomatic disarray was symptomatic of the deep hostility between the warring sides and the unpredictability injected by Trump, whose interventions since returning to the White House in January have often provoked dismay from Ukraine and its European allies.

While Zelenskiy waited in vain for Putin in Ankara, the Russian negotiating team sat in Istanbul with no one to talk to on the Ukrainian side. Some 200 reporters milled around near the Dolmabahce Palace on the Bosphorus that the Russians had specified as the talks venue.

The enemies have been wrestling for months over the logistics of ceasefires and peace talks while trying to show Trump they are serious about trying to end what he calls "this stupid war".

Hundreds of thousands have been killed and wounded on both sides in the deadliest conflict in Europe since World War Two. Washington has threatened repeatedly to abandon its mediation efforts unless there is clear progress.

Trump said on Thursday he would go to the talks in Turkey on Friday if it was "appropriate".

"I just hope Russia and Ukraine are able to do something. It has to stop," he said.

Russia accused Ukraine of "trying to put on a show" around the talks. Its lead negotiator said the Russians were ready to get down to work and discuss possible compromises.

Asked if Putin would join talks at some future point, Kremlin spokesman Dmitry Peskov said: "What kind of participation will be required further, at what level, it is too early to say now."

Russia said on Thursday its forces had captured two more settlements in Ukraine's Donetsk region. A spokeswoman for Russian Foreign Minister Sergei Lavrov pointedly reminded reporters of his comment last year that Ukraine was "getting smaller" in the absence of an agreement to stop fighting.

FIRST TALKS FOR THREE YEARS

Once they start, the talks will have to address a chasm between the two sides over a host of issues.

The Russian delegation is headed by presidential adviser Vladimir Medinsky, a former culture minister who has overseen the rewriting of history textbooks to reflect Moscow's narrative on the war. It includes a deputy defence minister, a deputy foreign minister and the head of military intelligence.

Key members of the team, including Medinsky, were also involved in the last direct peace talks in Istanbul in March 2022 - a signal that Moscow wants to pick up where those left off.

But the terms under discussion then, while Ukraine was still reeling from Russia's initial invasion, would be deeply disadvantageous to Kyiv. They included a demand by Moscow for deep cuts to the size of Ukraine's military.

With Russian forces now in control of close to a fifth of Ukraine, Putin has held fast to his longstanding demands for Kyiv to cede territory, abandon its NATO membership ambitions and become a neutral country.

Ukraine rejects these terms as tantamount to capitulation, and is seeking guarantees of its future security from world powers, especially the United States.

French Foreign Minister Jean-Noel Barrot said Zelenskiy had shown his good faith by coming to Turkey but there was an "empty chair" where Putin should be sitting.

"Putin is stalling and clearly has no desire to enter these peace negotiations, even when President Trump expressed his availability and his desire to facilitate these negotiations," he said.

Highlighting the level of tension between Russia and the U.S.-led alliance, Estonia said Moscow had briefly sent a military jet into NATO airspace over the Baltic Sea during an attempt by the Estonian navy to stop a Russian-bound oil tanker thought to be part of a "shadow fleet" defying Western sanctions on Moscow.

(Additional reporting by Can Sezer in Istanbul; Humeyra Pamuk, Tuvan Gumrukcu and John Irish in Antalya, Turkey; Huseyin Hayatsever in Ankara; Christian Lowe, Olena Harmash and Yuliia Dysa in Kyiv; Dmitry Antonov in Moscow; Andrius Sytas in Tallinn; Nandita Bose, Doina Chiacu and Susan Heavey in Washington; Lidia Kelly in Melbourne; Writing by Mark Trevelyan; Editing by Ros Russell)