KYIV, May 3 - Ukraine has formally closed its four Black and Azov sea ports, which Russian forces have captured, Reuters reported Monday citing the Ukrainian agriculture ministry.

The Azov Sea ports of Mariupol, Berdiansk and Skadovsk and the Black Sea port of Kherson were closed "until the restoration of control", the ministry said in a statement.

"The adoption of this measure is caused by the impossibility of servicing ships and passengers, carrying out cargo, transport and other related economic activities, ensuring the appropriate level of safety of navigation," it said.

All Ukrainian seaports have suspended their activity as a result of the Russian invasion in late February. Russian forces captured some ports and blockaded others.

Ukrainian President Volodymyr Zelenskiy said on Monday Ukraine could lose tens of millions of tonnes of grain due to Russia's control of Black Sea shipping, triggering a food crisis that will affect Europe, Asia and Africa.

"Russia does not let ships come in or go out, it is controlling the Black Sea," Zelenskiy told the Australian news programme 60 Minutes.

"Russia wants to completely block our country's economy."

Russia calls its intervention in Ukraine a "special operation" to disarm Ukraine and protect it from fascists. Ukraine and the West say the fascist allegation is baseless and that the war is an unprovoked act of aggression.

Ukraine, a major agricultural producer, used to export most of its goods by sea but has been forced to switch to export by train via its western border or via its small Danube river ports.

The ministry said last week Ukraine's grain exports had reached 45.709 million tonnes in the 2021/22, July-June season.

It said the volume included 763,000 tonnes exported in April but gave no comparative figures. Senior agriculture officials said this month that Ukraine exported up 300,000 tonnes of grain in March. (om/ez)