Ukrainian President Volodymyr Zelensky and US Defense Secretary Lloyd Austin shake hands during a meeting in Kiev . AFP
Following talks in Bucharest with Romanian Defense Minister Nicolae Ciuca, Austin said action was needed by littoral states of the Black Sea amid Russia "militarization" of the region.
"The security and stability of the Black Sea are in the US's national interest and critical for the security of NATO's eastern flank," he said.
"The region is vulnerable to Russian aggression and we've seen evidence of that by ongoing actions in eastern Ukraine (and) the occupations of parts of Georgia," he told reporters.
Austin was in Bucharest on the third stop of a tour of the region to build confidence among allies including Georgia and Ukraine, both of which hoping to join NATO, and to promote greater cooperation among military forces.
He will then take part in an in-person defense ministers summit at NATO headquarters in Brussels on Thursday and Friday.
The US has nearly 1,000 troops in Romania, a full member of NATO, which Austin says is good deterrence against potential Russian aggression.
Most are stationed at the southeastern Mihail Kogalniceanu airbase but a number are also there to support an Aegis missile defense system, the first installed on NATO's eastern flank.
Talks with Ciuca focused in part on how to advance current basic defense cooperation by the countries on the Black Sea rim.
"The Black Sea is and will continue to be an extremely important strategic center of gravity for European and Euro-Atlantic security," said Ciuca, speaking at a joint press conference.
Ciuca said that developing closer coordination with US and NATO forces in Romania, and deepening his own forces' capabilities, "represents the solution for countering Russia's assertiveness."
But US officials rejected speculation that the Pentagon would expand its troop presence in Romania or establish a permanent base in the country.
"Not only are we not increasing troop levels, but there are also no plans for permanent basing in Romania at this time," a senior US defense official said.