(Reuters) -Ukrainian President Volodymyr Zelenskiy hailed ties with Romania as a "factor of stability for Europe and beyond" on Tuesday as he visited the NATO member state for the first time since Russia's invasion last year.

Zelenskiy met President Klaus Iohannis, whose country is also in the European Union, at the start of a brief visit which the Ukrainian leader said would focus on security in the Black Sea region and also discuss developing an aviation coalition.

The Romanian port of Constanța has become Ukraine's main export route for grain via Ukrainian ports on the Danube River since Russia quit a deal in mid-July that had guaranteed safe shipments via three Ukrainian Black Sea ports.

"(The) Ukraine-Romania partnership is a factor of stability for Europe and beyond," Zelenskiy wrote on X, formerly known as Twitter, after starting talks with Iohannis.

In earlier comments on Twitter, he wrote: "Romania is a friend who came to our help on our darkest day and whose support gets stronger with time. I am certain this visit will be beneficial for both of our nations."

Zelenskiy, who was dressed in military khaki, said the agenda of his visit included "defense cooperation, Black Sea security, and economic cooperation .... with a focus on developing infrastructure and creating jobs in both nations."

He also said he would discuss further security cooperation including "developing aviation and other coalitions, strengthening Ukraine’s air defense, the Black Sea security architecture, and our relations with partners."

Romania has said it is considering opening a regional training hub for F-16 fighter jet pilots which would ultimately be available to its NATO allies and partners, including Ukraine.

The country, which shares a 650-km (400 mile) border with Ukraine, is host to a U.S. ballistic missile defence system and, as of last year, has a permanent alliance battlegroup stationed on its territory.

Romania has detected several drone fragments falling on its territory as Russia attacked Ukrainian port and grain infrastructure across the Danube River.

Defence officials in Bucharest told Reuters last month that the Romanian military was moving air defences closer to Danube villages and was adding more observation posts and patrols to the area.

(Reporting by Dan Peleschuk and Pavel Polityuk, Editing by Timothy Heritage)