"Talks are in fact going on now with Turkey and the UN (and) our representatives who are responsible for the security of the grain that leaves our ports," Ukraine's President Volodymyr Zelenskiy told a news conference alongside Swedish Prime Minister Magdalena Andersson.

"This is a very important thing that someone guarantees the security of ships for this or that country - apart from Russia, which we do not trust.

"We therefore need security for those ships which will come here to load foodstuffs."

Zelenskiy said Ukraine was working "directly" with UN Secretary General Antonio Guterres on the issue and that the organisation was "playing a leading role, not as a moderator".

News reports have suggested in recent weeks such talks would soon be taking place in Turkey.

Ukraine, one of the world's leading grain exporters, accuses Russia of blocking the movement of its ships, and Zelenskiy said 22 million tonnes of grain was stuck at the moment with a further harvest of about 60 million tonnes expected in the autumn.

Russia denies it is blocking any movement of grain and says Ukraine is to blame for the lack of movement, partly because of what it says are operations to lay mines in its ports.

Ukraine has also accused Russia of stealing grain from its warehouses and taking it out of the country - either to Russian-occupied areas, Russia itself or other countries.

A Turkish official on Monday said Turkey had halted a Russian-flagged cargo ship off its Black Sea coast and was investigating a Ukrainian claim that it was carrying stolen grain.

Australian Associated Press