The new government in Ukraine said Monday that Russia’s navy ordered two of its ships off the shores of Crimea to surrender.

Ukrainian defense ministry spokesman Maksim Prauta said that four Russian navy ships were blocking Ukraine’s anti-submarine warship Ternopil and the command ship Slavutych, which are currently near the port of Sevastopol.

The head of Russia’s Black Sea fleet gave the ships until 5 a.m. Tuesday to give up weapons and capitulate, Oleksiy Kirchkov, deputy commander of the Ternopil, told Ukraine’s Channel 5 by phone.

Western diplomats are seeking to calm tensions in Ukraine, with U.S. Secretary of State John Kerry arriving in Kiev Monday.

Western allies have condemned Russia’s threat to invade its Western-leaning neighbor, which analysts say has sparked the worst crisis between the West and Russia since the Cold War.

Merkel called on Putin to respect Ukraine’s territorial integrity and told him the intervention was a violation of a 1994 Budapest memorandum on security assurances in which Russia committed itself to respecting the independence and sovereignty of Ukraine in its existing borders, as well as the 1997 treaty on the Russian Black Sea fleet, based in Crimea.

The memorandum was signed by Britain, Ukraine, Russia and the United States.

Earlier Sunday, Ukraine’s newly-installed Prime Minister Arseniy Yatsenyuk appealed to Moscow to move its troops back, saying that the region was “on the brink of disaster. There was no reason for the Russian Federation to invade Ukraine.”