The UN secretary-general, Antonio Guterres, and President Erdogan of Turkey have accepted an invitation from President Zelensky to meet Thursday at Lviv in Ukraine to review the deal allowing Ukrainian grain to be shipped to world markets and to discuss ways to end the six-month-old war.

The UN spokesman, Stephane Dujarric, told reporters that he expects “the need for a political solution” to the war to be raised during the meeting in the western Ukrainian city, which is close to the Polish border.

Mr. Dujarric also said that had no doubt the three leaders will also discuss the situation at the Russian-controlled Zaporizhzhia nuclear power plant, where Russia and Ukraine have accused each other of shelling.

He added that they also will likely talk about a UN fact-finding mission to investigate the killings at the Olenivka prison in a separatist region of eastern Ukraine. The warring nations accuse each other of responsibility.

The trilateral meeting comes after the signing of an international agreement in Istanbul on July 22 clearing the way for Ukraine to export 22 million tons of corn and other grain stuck at its Black Sea ports and in silos since Russia invaded the country in February. A separate memorandum between Russia and the UN signed the same day was aimed at clearing roadblocks to its shipments of food and fertilizer to world markets.

Mr. Guterres first proposed the grain deal to President Putin and Mr. Zelensky at separate meetings in Moscow and Kyiv in late April. Mr. Dujarric said the secretary-general’s trip to Ukraine is “a chance for him just to see first-hand the results of an initiative … that is so critically important to hundreds of millions of people.”

After the three-way meeting, and likely bilateral talks between Messrs. Erdogan and Guterres, the UN chief will travel to Odessa, one of the three Ukrainian ports now operating to ship grain, on Friday, Mr. Dujarric said. He will then travel to Istanbul on Saturday to visit the center coordinating the Black Sea shipping, which includes the four parties to the deal — Ukraine, Russia, Turkey, and the United Nations.

Russia was not invited by Mr. Zelensky to the Lviv meeting.

Mr. Dujarric said the secretary-general had “a very good conversation” Monday with the Russian defense minister, Sergei Shoigu, which touched on the grain shipments from both Ukraine and Russia.

During the phone call, Messrs. Guterres and Shoigu also discussed “the conditions for the safety operations of the Zaporizhzhia nuclear power plant” and a fact-finding mission to the Olenivka prison, Mr. Dujarric said. Whether prospects for a ceasefire were also discussed was not immediately clear.

The war and a halt to all Ukrainian grain shipments and most Russian shipments of grain and fertilizer added significantly to the global food crisis because both countries are major suppliers to world markets.

Developing countries have been especially hard-hit by supply shortages and high prices. Even though ships are now leaving Russia and Ukraine and some prices have dropped, the food crisis has not ended.