Military personnel, believed to be Russian servicemen, walk outside a Ukrainian military unit in the village of Perevalnoye.
The United States is suspending all military engagements with Russia over the crisis in Ukraine, including military exercises and port visits, the Pentagon said on Monday.
"We call on Russia to de-escalate the crisis in Ukraine and for Russian forces in Crimea to return to their bases, as required under the agreements governing the Russia Black Sea Fleet," Pentagon spokesman Rear Admiral John Kirby said.
Russian forces began moving troops into Ukraine's Crimea region by ferry on Monday after seizing control of the border post on the Ukrainian side of the waterway, Ukraine's border guards said.
Russians who seized the isolated Black Sea peninsula have been surrounding the ferry terminal for days but until now had not taken control of Ukraine's border guard station.
A border guard spokesman said Russian troops seized the checkpoint after the border guards tried to stop two buses carrying seven armed men, and the next ferry brought three truckloads of soldiers across.
Earlier on Monday, the Ukrainian border guards said they had seen Russia assembling an armored column on its side of the 4.5 km (2.7 mile) wide Kerch strait that separates the Crimea peninsula from southern Russia.
Russian troops have seized Crimea and President Vladimir Putin has declared Moscow has the right to intervene in Ukraine to protect Russian citizens in Ukraine, sparking the biggest crisis between Russia and the West since the Cold War.
Earlier on Monday, Russia's Black Sea Fleet denied that it had issued an ultimatum to Ukrainian forces in Crimea to surrender by 5 a.m. on Tuesday or face an assault, Interfax news agency quoted an official at the fleet's headquarters as saying.
Russia's Black Sea Fleet has a base in Crimea and Moscow has effectively established control over the peninsula, which is part of Ukraine.
Interfax quoted an unnamed source in the Ukrainian Defense Ministry earlier on Monday as saying a deadline to surrender at 0300 GMT had been set by the Black Sea Fleet's commander.
The same news agency later quoted an unnamed representative at the fleet's headquarters as saying no assault was planned, adding: "This is complete nonsense."
The ultimatum, Interfax initially said, was issued by Alexander Vitko, the fleet's commander.
"If they do not surrender before 5 a.m. tomorrow, a real assault will be started against units and divisions of the armed forces across Crimea," the agency had quoted the ministry source as saying.
The seizure of Crimea has created the greatest confrontation between Russia and the West since the collapse of the Soviet Union in 1991, an event Putin has described as the worst geopolitical catastrophe of the 20th century.
Moscow has said it is protecting the lives of Russian-speakers, who are in the majority in Crimea and numerous in much of eastern Ukraine, and appears to be calculating that the West cannot afford to risk a wider conflagration by taking anything approaching military action.
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