The UN said Friday it is leading intense negotiations with Russia to unblock Ukrainian ports and release tens of millions of tonnes of grain to avert a global food crisis.

One hundred days into Russia's invasion of Ukraine, the UN crisis coordinator for the war-torn country, Amin Awad, stressed the high stakes of the "very, very complex" talks to try and end the impasse.

Ships loaded with grain remain blocked in Ukraine, which before February was considered a global breadbasket as a leading exporter of corn, wheat and sunflower seeds, feeding 400 million people around the world last year.

The talks are being led by UN aid chief Martin Griffith and Rebeca Gynspan, who heads the UN trade and development agency, Awad said via video link to reporters in Geneva.

The UN has warned that especially African countries, which imported more than half of their wheat consumption from Ukraine and Russia, face an "unprecedented" crisis caused by the conflict.

Food prices in Africa have already exceeded those in the aftermath of the 2011 Arab springs and the 2008 food riots.

Putin has said that Moscow is ready to look for ways to ship grain stuck in Ukrainian ports but has demanded the West lift sanctions.

But Awad highlighted that pressure is also being put on Russia from some of its allies feeling the pinch.

"There is a lot of shuttling between Moscow and other countries that have concerns," he said.

On Friday, Russian President Vladimir Putin met the head of the African Union, Senegalese President Macky Sall, at his Black Sea residence in Sochi.

At the opening of those talks, Sall told Putin to "become aware" that African countries "are victims" in the Ukraine conflict.

Food prices have shot up in Africa
Food prices have shot up in Africa AFP / ASHRAF SHAZLY

Awad highlighted that Russia "has alliances in the South", stressing that some of the impacted countries could help sway the situation.

"I am optimistic that something could give in, something could be made," he said, voicing hope that we could "see a breakthrough".

But, he stressed, the negotiations are "very complex" and "happening on many tracks."

The UN's World Food Programme said unblocking the ports would have a huge impact.

"The Black Sea ports are as it were the silver bullet when it comes to avoiding global famines, global hunger," WFP's emergency coordinator in Ukraine Matthew Hollingworth told reporters.

He said while efforts are underway to reopen the ports, the UN and others are also looking at other options for getting the desperately-needed grain out of Ukraine, including by truck, train or through ports in neighbouring countries.

Such options would however mean "dribbling out 1-1.5 million tonnes," he said, highlighting that while that may sound like a lot, "it is nothing when this country was pushing five million tonnes out a month prior to this war."

Awad agreed, pointing to a range of challenges with moving grain by truck or rail.

"It does have to be really be a maritime movement to support 50 to 60 million tonnes of food out," he said.