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KIEV, Ukraine – The besieged new government of Ukraine accused Russian forces of a major escalation in pressure over control of the Crimea on Monday night, saying the Russians 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 emergency session since Friday, the Ukraine ambassador, Yuriy Sergeyev, distributed a three-page letter asserting that Russia had sent 16,000 troops into the Crimean peninsula since Feb. 24. The troops, Sergeyev wrote, had moved to “seize, block and control crucial governmental and military objects of Ukraine in Crimea.”

The Interfax-Ukrainian news agency quoted an unidentified Ukrainian Defense Ministry official as saying Russia’s Black Sea Fleet commander had set a deadline of 5 a.m. today – 10 p.m. Monday Eastern time – for Ukrainian forces stationed in Crimea to lay down their weapons. Russia’s Interfax news agency said the Black Sea Fleet had no such plans.

The conflicting reports only further served to worsen tensions in the Ukraine crisis, which has grown drastically in scope within the past few weeks to a new confrontation between Russia and the West reminiscent of low points in the Cold War.

Obama, who spent much of the weekend working on the crisis, issued a new warning on the consequences to the Kremlin.

“What we are also indicating to the Russians is that if, in fact, they continue on the current trajectory that they’re on, that we are examining a whole series of steps – economic, diplomatic – that will isolate Russia and will have a negative impact on Russia’s economy and its status in the world,” he said.

EU foreign ministers, condemning Russia’s actions, called on Moscow to return its troops to their bases. EU heads of government will meet in an emergency summit meeting Thursday to discuss further steps. The foreign ministers threatened to freeze visa liberalization and economic cooperation talks with Russia and boycott a Group of 8 summit in Sochi if Moscow did not take steps to “de-escalate” the situation by the Thursday summit.

NATO also called its second emergency meeting on Ukraine in response to a request from Poland under Article 4 of the NATO treaty, which concerns threats to a member state’s security and independence.

Visiting the new government in Kiev, British Foreign Secretary William Hague urged Russia to pull back its forces in Crimea or face “significant costs,” echoing comments made by Obama and U.S. Secretary of State John F. Kerry, who was due here today.

Hague also emphasized diplomacy. “The world cannot just allow this to happen,” he told the BBC. “The world cannot say it’s OK in effect to violate the sovereignty of another nation in this way.”

The Russian foreign minister, Sergei V. Lavrov, responded that Russia was only protecting its interests and those of Russian citizens in Ukraine.

The use of Russian troops is necessary “until the normalization of the political situation” in Ukraine, Lavrov said at an opening of a monthlong session of the U.N. Human Rights Council in Geneva. “We are talking here about protection of our citizens and compatriots, about protection of the most fundamental of the human rights – the right to live, and nothing more.”

With the new Kiev government confronted with the loss of Crimea and a worsening economic situation, a team from the International Monetary Fund was scheduled to arrive today for a 10-day investigation of the true state of Ukraine’s finances.

Moscow suspended its offer of bond purchases when President Viktor Yanukovych was ousted more than a week ago.

Although Crimea was relatively calm Monday, Russian forces tightened their grip on key military bases and other security facilities throughout the peninsula.

At the Azov-Black Sea regional headquarters of the border police in Simferopol, the Crimean regional capital, a half-dozen men in plainclothes, some wearing face-masks but carrying military radios, stood guard outside the front door, where the glass had been smashed out during a siege of the building Sunday.

At least three large Russian troop carriers were parked outside the building. Although most identifying markers had been removed, one of the trucks bore a black Russian military license plate carried by vehicles attached to the headquarters of the Black Sea Fleet in Sevastopol.

The men at the door refused to identify themselves and did not permit visitors to enter the building.

In Donetsk, in eastern Ukraine, Yanukovych’s native region, a large pro-Russian demonstration led to some violence. About 1,000 pro-Russian demonstrators occupied the first floor of the regional government building that has been flying the Russian flag for days.

In a statement Monday night, Ukraine’s acting president, Oleksandr Turchynov, said officers in Crimea had high morale and would defend themselves if necessary.

“Nobody will ever give up Crimea,” he said.