KYIV - Dozens of ships are backed up around critical Danube arteries close to Ukraine's river gateways days after Russian drone attacks on the country's ports, shipping data showed on Tuesday.

The river and its mouth are Ukraine's last remaining waterborne grain export route.

Russia has attacked the agricultural and port infrastructure of Ukraine, one of the world's top grain exporters, in recent weeks after refusing to extend a year-old safe passage grain corridor brokered by the United Nations and Turkey - effectively shutting Ukraine's Black Sea ports.

These included the Danube ports of Izmail and Reni, shipping sources said.

At least 30 ships had dropped anchor around Musura Bay in the Black Sea, which leads into a channel that links up with Izmail further along the waterway, tracking data from analytics company MarineTraffic showed on Tuesday.

There were at least 20 ships anchored leading up to Izmail.

"It is impossible to transport the entire volume by river or rail," Ukraine's First Deputy Agriculture Minister Taras Vysotskiy told national television.

"In order to export all the grain, the Black Sea ports need to be reopened."

In addition, there were at least 20 commercial ships waiting close to the Romanian port of Constanta, the MarineTraffic data showed. Many of the vessels had reported their destination as Romanian ports.

"The two Ukrainian ports continue to reel from the Russian attacks. It took time to reorganise, re-establish communication," Florin Uzumtoma, navigation director for Romania's Danube administration agency, said.

"Not all the ships clustered on the water want to enter Romania. Some of them are just waiting in safe waters. There are anchor spots on the Danube and they are just waiting there until they are ready to meet all the formalities to cross."

Some marine insurers have started to restrict the amount of cover they are providing for Danube shipments after pausing insurance provisions for Ukraine ports that were part of the grain corridor initiative, insurance sources said.

"This is not only about insurance, but also about overall shipping activities in the area for shipowners who might consider that risks for their crew and their assets are just too high," one underwriter said, asking not to be named.

Before Russia pulled out of the safe passage corridor, the Danube ports accounted for around a quarter of Ukraine's grain exports.

Ukraine, which needs to boost exports of its bumper grain crop, has started work to expand alternative export opportunities by organising raid transshipment near the mouth of the Danube River, seaports authorities said on Tuesday.

(Reporting by Jonathan Saul London, Pavel Polityuk in Kyiv and Luiza Illie in Bucharest, additional reporting by Carolyn Cohn in London; editing by Barbara Lewis)