The Russian military has increased its activity in the Black Sea, according to a U.S. senior defense official.

“We have seen some increased naval activity in the northern Black Sea,” the official told reporters Monday morning, noting that Russia has “a little bit more than a dozen warships” in the area. Despite the increased activity, the official said it is not clear whether an amphibious assault on Odesa, a city along the southern coast of Ukraine along the Black Sea, is “imminent.”