BUCHAREST, Jan 11 (Reuters) - Romania will remain Ukraine's largest alternative export route for grains and other goods in addition to Kyiv's own Black Sea corridor, a senior U.S. State Department official said on Thursday.

Ukraine is one of the world's biggest grain exporters. It began using Romania's Black Sea port of Constanta after Russia's full-scale invasion in February 2022 halted shipments from its own Black Sea ports.

It exported 14 million metric tons of grain through Constanta in 2023, roughly 40% of the port's entire grain shipments for the year, up from 8.6 million tons in 2022.

However, transit volumes fell in the second part of the year after Russia repeatedly struck Ukraine's river ports across the Danube from European Union and NATO member Romania.

Ukraine also created a shipping corridor from its own ports in August, which hugs the western Black Sea coast near Romania and Bulgaria, shortly after Russia withdrew from the U.N.-brokered Black Sea grain export deal and threatened to treat all vessels as potential military targets.

In a briefing on Thursday the U.S. State Department's Assistant Secretary for European and Eurasian Affairs Jim O'Brien said Ukraine exported about 7.5 million tons of grain in December, mostly through its own corridor but also through Constanta.

"I see going forward some kind of a balance like that," O'Brien told reporters.

"To recover, Ukraine needs to export. I think, just holding the levels of export we have now, about seven million tons of grain, and another million or so tons of other items a month, would mean more than $25 billion a year in GDP for the Ukrainian economy. That's five to six billion dollars in tax revenue."

O'Brien and the European Commission's director general for mobility and transport have been holding regular meetings with Romanian, Moldovan and Ukrainian officials to find ways to boost grain transit capacity. (Reporting by Luiza Ilie; Editing by Kirsten Donovan)