Kyiv and Moscow on Friday penned a landmark agreement with Turkey and the United Nations Friday to unblock Ukraine's Black Sea grain exports after a Russian blockade raised fears of a global food crisis.

In their first major deal after nearly five months of fighting, Russian and Ukrainian delegations signed individual agreements with Ankara and the UN in the wake of several rounds of laborious negotiations.

Ukraine had warned ahead of the signing that any Russian "provocations" around its encircled Black Sea ports would be met with a swift military response and refused to pen the same papers as Moscow.

"Today, there is a beacon on the Black Sea -- a beacon of hope, a beacon of possibility, a beacon of relief," UN Secretary General Antonio Guterres said at the signing ceremony in the lavish Dolmabahce Palace on the Bosphorus Strait.

The deal agreed through UN and Turkish mediation establishes safe corridors along which Ukrainian ships can come in and out of three designated Black Sea ports in and around Odessa.

And both sides also pledged not to attack ships on the way in or out.

"It will bring relief for developing countries on the edge of bankruptcy and the most vulnerable people on the edge of famine," Guterres said.

The five-month war, which has displaced millions and left thousands dead, is being fought across one of Europe's most fertile regions by two of the world's biggest grain producers.

Up to 25 million tonnes of wheat and other grain have been blocked in Ukrainian ports by Russian warships and landmines Kyiv has laid to avert a feared amphibious assault.

"Today's Istanbul agreement is a step in the right direction. We call for its swift implementation," tweeted the bloc's foreign policy chief Josep Borrell. (AFP)