KIEV, Ukraine — The embattled new government of Ukraine accused Russian forces of a major escalation in military pressure over control of the Crimean Peninsula on Monday, saying Russia had deployed 16,000 troops in the region over the last week and had demanded that Ukrainian forces there surrender within hours or face armed assault.

Russia denied it had issued any ultimatum but was clearly moving to strengthen its grip on Crimea, brushing aside new admonitions from President Obama and European leaders of economic punishment and isolation.

At the United Nations, where the Security Council met for the third time in an emergency session since Friday, the Ukrainian ambassador, Yuriy Sergeyev, distributed a three-page letter asserting that the Russians had sent 16,000 troops into the Crimean Peninsula since Feb. 24.

The troops, Mr. Sergeyev wrote, had moved to “seize, block and control crucial governmental and military objects of Ukraine in Crimea.”