The leaders of Turkey and the UN were due to visit Ukrainian President Volodymyr Zelenskyy on Thursday as they aim to build on their Black Sea grain deal to pursue the greater prize of a ceasefire between Ukraine and Russia.
Turkish President Recep Tayyip Erdogan and UN Secretary General Antonio Guterres will meet Mr Zelenskyy for talks in the western city of Lviv, their offices said.
The three men will tackle issues including the situation at the Zaporizhzhia nuclear power plant, the movement of cargo ships from Ukraine's Black Sea ports, a recent attack on a prisoner of war camp and the wider issue of bringing peace after six months of war.
Bilateral meetings are also expected between Mr Zelenskyy and each of his two visitors, with Mr Erdogan travelling to Ukraine for the first time since Russia invaded in February.
Mr Guterres will continue to Odesa on Friday to visit one of the ports being used to export Ukrainian food, before going to the joint co-ordination centre in Istanbul that was established as part of the grain deal.
A spokesman for Mr Guterres said the leaders would discuss the need for a political solution and the secretary general's desire to "lower the temperature as much as possible".
Mr Erdogan’s office said the leaders would talk about “ending the Ukraine-Russia war through diplomatic avenues”.
Such talk has sometimes frustrates Ukrainian allies who say the country should push for an outright military victory rather than settling for unhappy peace.
But Mr Erdogan said after a signing ceremony for the grain deal in Istanbul that it was only through a political solution that the fighting would be brought to an end.
“I hope from the bottom of my heart that this joint step we are taking today with Russia and Ukraine constitutes a new turning point that would revive the hopes for peace,” he said at the time.
Mr Zelenskyy said only that "we will work to get the necessary results for Ukraine" at the talks.
But he said Ukrainian diplomats and nuclear scientists were in constant contact with the UN's atomic energy agency to arrange a visit to Zaporizhzhia, where he said Russia should vacate the area.
Russia and Ukraine accuse each other of shelling the plant and of blocking international inspections, creating a perilous situation which Mr Guterres said could lead to disaster.
Almost six months of fighting have left Russia in control of large parts of eastern and southern Ukraine, but the Kremlin’s forces have so far failed to topple Mr Zelenskyy’s government and are now dealing with Ukrainian counter-attacks.
Two civilians were killed and seven wounded in shelling by Russian forces in the past 24 hours, said Pavlo Kyrylenko, governor of Ukraine's eastern Donetsk region.
Early peace talks in March went quiet when evidence emerged of potential war crimes by Russian troops near Kyiv, but the grain deal brokered by the UN and Turkey marked the first major diplomatic breakthrough of the war.
Ukraine signalled in the early peace negotiations that it was willing to discuss a form of neutrality with security guarantees from named countries instead of its long-held ambition of Nato membership.
However, Mr Zelenskyy has rejected the idea of conceding territory, and a senior aide Mykhailo Podolyak last week said there were no negotiations at present because “it is currently impossible to find common ground”.
Turkey has positioned itself as a mediator during the conflict, having criticised the invasion of Ukraine but sought to keep the Kremlin on reasonably friendly terms.
Mr Erdogan twice met Russian President Vladimir Putin in recent weeks and said at their last meeting in Sochi that he hoped to “open a different page” in relations between their two countries.
Ukraine crisis - in pictures
The UN has also reached out to both sides. Mr Guterres’s spokesman reported a “very good conversation” this week between the secretary general and Russian Defence Minister Sergei Shoigu.
He said the first shipment of Ukrainian wheat run by the UN’s World Food Programme had left a Black Sea port on Tuesday, as humanitarian agencies try to ease a global hunger crisis caused by the war in Ukraine.
The spokesman called it “another important milestone in efforts to get much-needed Ukrainian grain out of the conflict-hit country back into global markets and to countries worst-affected by the global food crisis”.
Mr Guterres was also expected to raise the attack on Olenivka prison late last month, which Ukraine and the US described as a deliberate act of Russian shelling against prisoners of war.
The UN has sent a fact-finding mission. Russia blamed Ukraine but the US said Moscow was planting evidence to deflect blame.