MOSCOW: Russia has suspended movement of commercial vessels in the Azov sea until further notice, but kept its ports in the Black Sea open for navigation, its officials and five grain industry sources said on Thursday.

Russian President Vladimir Putin authorised "a special military operation" against Ukraine on Thursday to eliminate what he called a serious threat, saying his aim was to demilitarise Russia's southern neighbour.

Russia, the world's largest wheat exporter, mainly ships its grain from ports in the Black Sea.

The Azov sea is home to shallow water ports of smaller capacity.

"All ships are on 'stop' (in the Azov sea)," a grain industry source told Reuters on condition of anonymity.

Chicago's most active wheat contract is up 5.6% at its highest since mid-2012.

Russia and Ukraine account for 29% of global wheat exports, 19% of world maize (corn) supplies, and 80% of world sunflower oil exports.

Russia produced 76 million tonnes of wheat last year and is expected by the US Department of Agriculture to export 35 million tonnes in the July-June season, 17% of the global total.

Russia supplies wheat to all the major global buyers. Turkey and Egypt are the largest importers.

Russian ground forces cross into Ukraine: Kyiv

Ukraine asked Turkey to close the Bosphorus and Dardanelles straits to the Russian ships, the Ukrainian ambassador to Ankara said earlier on Thursday.

There has been no reaction yet to Ankara's request.