Some analysts worry that President Vladimir V. Putin’s engagement in diplomacy is merely buying time for Russia’s military to make final preparations for an invasion of Ukraine.

On Tuesday, the Russian defence ministry said in a statement that six large landing craft from the Russian navy’s Baltic and Northern Fleets, capable of carrying thousands of troops, had been dispatched to the waters off Ukraine in the Black Sea, raising concerns that they could open a new vector of attack.

The statement said that the ships were scheduled to take part in planned military exercises. Military analysts, however, say that combining the ships with forces already deployed to the Black Sea region would provide a significant amphibious assault force capable of threatening a large area of the coastline of Ukraine, which has only a limited coastal defence system.

In particular, analysts say, Ukraine lacks long-range anti-ship missiles with which to counter Russian forces in the Black Sea and Sea of Azov, a small body over water over which Russia’s has exerted naval dominance since its annexation of Crimea in 2014.