Ukrainian President Volodymyr Zelensky on Saturday called for international pressure to end a Russian naval blockade of Black Sea ports that has choked off his country's grain exports, threatening a global food crisis.

Before the Russian invasion, Ukraine was the world's top producer of sunflower oil and a major wheat exporter, but millions of tonnes of grain exports remain trapped due to the blockade.

The United Nations and some countries are pushing for a maritime corridor to be opened up to allow exports to resume.

“The world will face an acute and severe food crisis and famine, in many countries of Asia and Africa,” Zelensky said in a video address to the Shangri-La Dialogue security summit in Singapore.

“The shortage of foodstuffs will inexorably lead to political chaos, which can result in the (collapse) of many governments and the ousting of many politicians,” he told delegates, including Pentagon chief Lloyd Austin and China's defence minister.

“This looming threat is plain to see by just looking at the skyrocketing prices of basic products in the world markets and in certain countries. This is the direct consequence of the acts of the Russian state.”

Zelensky urged the international community to “restore the full might of the international law” that existed before the February 24 invasion.

Kyiv is in discussion with the UN, Turkey and other countries to open a way to allow the grain exports, and Zelensky said the talks are focused on the “format” of the corridor.

Russian Foreign Minister Sergei Lavrov and his Turkish counterpart held talks this week in Ankara on securing safe passage for Ukrainian grain exports, but the discussions made little headway.

Zelensky said Ukraine was currently exporting more than two million tonnes of grain a month via rail but this was not enough.

He accused Russia of seeking to push up grain prices higher, adding it had done the same with energy.

Russia's invasion sparked worldwide condemnation and a barrage of sanctions. After being repelled from Kyiv and other parts of the country, it is focusing its offensive on the eastern Donbas region.