Russia's defense minister said on Tuesday the Ukrainian ports of Berdyansk and Mariupol, seized by Russian forces, have been de-mined and are ready to resume grain shipments.

"The de-mining of Mariupol's port has been completed. It is functioning normally, and has received its first cargo ships," Sergei Shoigu said in televised comments.

Agricultural exports from southern Ukraine have been blocked since Russia invaded the country in late February, driving grain prices higher.

Ukraine has accused Russia of stealing vital grain supplies - claims U.S. Secretary of State Antony Blinken has called "credible" - while Moscow says Western sanctions are to blame for the situation, which has threatened to trigger a global food crisis.

The United Nations is working on plans with Kyiv and Moscow for how to restart grain exports from Ukrainian ports, with Turkey possibly set to provide naval escorts to ensure safe passage out of the Black Sea.

Shoigu also said on Tuesday Russian armed forces had created the "necessary preconditions for the full resumption of railway traffic between Russia, the Donbas, Ukraine and Crimea" and had started delivery of cargo to the Ukrainian cities of Mariupol, Berdyansk and Kherson" on 1,200 km (750 miles) of reopened railway tracks.

Creating a so-called "land corridor" between Russia and the Crimean peninsula, annexed by Moscow in 2014, has been a major part of Russia's strategy since the start of its offensive.

Some 6,489 Ukrainian military personnel have surrendered to Russian forces since the start of what Russia calls its "special military operation," including 126 in the past five days, Shoigu added.