BUCHAREST - Ukraine has sent some 601,115 tonnes of grains to the Romanian Black Sea port of Constanta since Russia invaded on Feb. 24, with an additional 120,294 tonnes en route, the port said on Thursday.

The grains it has sent by barge, train and truck to the Romanian port amount to roughly 3% of the 20 million tonnes it needs to move before the new harvest starting at the end of July to avoid bottlenecks and forestall a global food crisis.

With Ukraine's sea ports blocked since the war started more than three months ago, the world's fourth-largest grain exporter has been forced to send shipments by train via its western border or through its small Danube river ports into Romania.

The United Nations plans to set up a sea corridor for Ukraine to ship out grains through its own ports, with help from Turkey, but there were major challenges to such a deal.

In Constanta, more than 262,000 tonnes of Ukrainian grains have already left aboard 15 ships, the port told Reuters, with more than 339,000 tonnes in silos and an additional 120,294 tonnes scheduled to arrive primarily by barge from Ukraine's Danube ports of Reni and Izmail.

Last year, Constanta's port shipped a record high 25.2 million tonnes of grain from Romania and its landlocked neighbours.

In May, port operator Comvex manager Viorel Panait estimated that operators could handle an additional 20% on top of last year's volumes, or roughly 5 million tonnes.

Comvex, which has handled most of the Ukrainian grains so far, operates Europe's fastest-loading grain terminal, which can process up to 70,000 tonnes per day.

(Reporting by Luiza Ilie; Editing by Edmund Blair)