The United States will provide an additional $10 million in military financing to Ukraine to bolster its navy after Russia captured three Ukrainian vessels at sea last month, in a “dangerous escalation,” the State Department said on Friday.

The move comes after Lithuania and the United Kingdom increased their security assistance to Ukraine, the department said, following the November 25 attack near the Kerch Strait.

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“The United States calls on Russia to immediately return to Ukraine the seized vessels and detained Ukrainian crews, to keep the Kerch Strait and the Sea of Azov open to ships transiting to and from Ukrainian ports, and to respect Ukraine’s sovereignty and territorial integrity,” the State Department added.

Meanwhile, more than a dozen SU-27 and SU-30 fighter jets which Russia is deploying to boost its air force arrived in Crimea on Saturday.

A Reuters witness saw the jets land at Belbek air base in Crimea, which was annexed by Russia in 2014 after Moscow-leaning Ukrainian President Viktor Yanukovich fled Kiev following street clashes and violent protests.

Tensions between Moscow and Kiev have risen in the past weeks after Russia seized three Ukrainian navy ships and their crews on Nov. 25 in an incident which Moscow and Kiev have blamed on each other.

Russian Foreign Minister Sergei Lavrov has alleged Ukraine was preparing “a provocation” near Crimea before the end of the year.

Russia opened fire on the Ukrainian ships and then seized them and their crews near Crimea - which Russia annexed from Ukraine in March 2014. The Kerch Strait is the only outlet to the Sea of Azov and controls access to two major Ukrainian ports.

The incident prompted U.S. President Donald Trump to call off a meeting with Russia’s Vladimir Putin in Argentina to signal Washington’s disapproval of Russian behavior in the naval clash with Ukraine.