As separate meetings between the United States, Russia and Ukraine entered the third day on Tuesday in Riyadh, Saudi Arabia, officials and experts said the talks could pave the way for a ceasefire and ultimately end the protracted conflict only if both sides are sincere and willing to make compromises.

Reviving the Black Sea deal, which was signed in 2022 to ensure the safe commercial shipping of grain crops and other items, and from which Russia withdrew a year later, was among the top topics said to be discussed at the meetings.

US negotiators expressed the hope that a resurrection of the deal would lead to a 30-day ceasefire on the ground and in the air, which could then facilitate a more permanent cessation of hostilities between Russia and Ukraine.

Russia said on Tuesday that it is willing to strike a new agreement on the safety of shipping in the Black Sea only if the US asked Ukraine to respect it, Saudi media group Al Arabiya News reported.

Moscow will not tolerate any ambiguity regarding proposals related to the Black Sea, Russian Foreign Minister Sergey Lavrov was quoted as saying in a televised statement by Russian news agency TASS.

US and Ukrainian negotiators held two rounds of talks on Sunday and Tuesday. The second round of talks has ended, a source in the Kyiv delegation said on Tuesday. "All details will be announced later," the Ukrainian source added.

US and Russian officials held discussions for 12 hours on Monday. Despite such marathon diplomatic efforts, a joint statement on a partial ceasefire was not adopted.

"The fact that they sat for 12 hours and apparently agreed on a joint statement, which was not adopted though because of Ukraine's position, is very typical and symptomatic," said Vladimir Chizhov, a senior Russian senator.

Kremlin spokesman Dmitry Peskov said on Tuesday that Moscow was "analyzing the outcomes" of the talks in Riyadh. The talks were "technical" and their content would not be made public, he said, adding that contacts between Russia and the US would continue.

Grigory Karasin, chairman of Russia's Federation Council Committee on International Affairs, said: "(We) discussed everything and the dialogue was detailed and complex, but quite useful for us and for the Americans. We discussed numerous issues."

US President Donald Trump told reporters that Washington expects to sign a deal with Kyiv soon that will give US companies access to Ukrainian critical minerals.

Experts have welcomed this week's developments and said the negotiations are heading toward a positive direction.

Radhika Desai, a professor in the department of political studies at the University of Manitoba in Canada and a visiting fellow at the London School of Economics and Political Science, said the talks have so far been dedicated to "technical details".

"I'm not expecting any major breakthroughs at this point. … Every side wants a certain type of optics," she said, adding that Trump now faces a certain amount of pressure.

"He needs to make it look as though he's not conceding everything to the Russians. He also needs to make it look as if Russia will make some concessions or has made some concessions."

As the talks continued in Riyadh, Ukraine also began lobbying European nations in the hope of securing an extension to an agreement that has allowed the country to trade without restrictions with the European Union.

With the deal set to expire on June 5 and with some EU member nations reluctant to renew it, Ukrainian Finance Minister Serhiy Marchenko has said his country's economy would be badly hit if the deal is allowed to lapse.

An unnamed Ukrainian official told the Financial Times: "The message is very simple: Our producers need predictability over exports, (the) EU can't start negotiations one week before the current regulation expires."