Russian forces further tightened their hold on Crimea early Sunday morning, capturing a Ukrainian border guard post in the west of the region, trapping and holding around 30 personnel inside.
The Chernomorskoye base on the western edge of the Back Sea peninsula was taken over without bloodshed at around 4am GMT, a border guard spokesman said.
He added that Russian forces now controlled a total of 11 border guard posts across Crimea.
The seizure comes a day after an unarmed Ukrainian border patrol plane came under fire while flying near the administrative border with Russian-occupied Crimea on an observation mission. No one was hurt.
Russian forces began taking control of the south Ukraine autonomous region 11 days ago - soon after Ukraine's pro-Russian President Viktor Yanukovich's fled the country.
Ukrainian troops are now trapped in a number of bases across Crimea, having offered no resistance - so far Kiev has held back from any action that might provoke a violent response from its neighbour. Ukraine's military, with barely 130,000 troops, would be no match for Russia's.
In pictures: Ukraine crisis
In pictures: Ukraine crisis
1/12 Ukraine crisis
People shout slogans during a pro Russian rally at a central square in Donetsk. Pro Russian activists continued to gather on Saturday in the eastern Ukrainian city of Donetsk, as Russia was reported to be reinforcing its military presence in Crimea.
2/12 Ukraine crisis
In the same pro Russian rally, demonstrators show their support. Ukraine's ambassador to Russia and a deputy Russian foreign minister held a "cordial" meeting on Saturday, Moscow said, without giving details of any discussion of Russian-occupied Crimea.
3/12 Ukraine crisis
Crimean ethnic tatars stand on the roadside as Russian troops move towards to Simferopol in the settlement of Kok-Asan, some 70 kilometres from Simferopol in Crimea.
4/12 Ukraine crisis
Russian troops stand on a roadside in the settlement of Opytnoye, some 70 kilometres from Simferopol.
5/12 Ukraine crisis
Armed members of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" march before the swearing-in ceremony in Simferopol, Ukraine. Some 30 men armed with automatic weapons and another 20 or so unarmed, were sworn in at a park in front of an eternal flame to those killed in World War II.
6/12 Ukraine crisis
A group of Cossacks march past a statue of Soviet revolutionary leader Vladimir Lenin in Simferopol as tensions in the area continue to rise.
7/12 Ukraine crisis
An armed member of the first unit of a pro-Russian armed force, dubbed the "military forces of the autonomous republic of Crimea" signs the oath during the swearing-in ceremony in Simferopol,
8/12 Ukraine crisis
9/12 Ukraine crisis
Ukrainian soldiers load their armed personnel carriers (APCs) into boxcars in the western Ukrainian city of Lviv. Pro-Kremlin militia fired warning shots as unarmed foreign observers tried to enter Crimea on the 8th.
10/12 Ukraine crisis
An abandoned naval ship sunk by the Russian navy to block the entrance is seen in the Crimean port of Yevpatorya on March 8th.
11/12 Ukraine crisis
Ukrainian sailors stand guard on top of the Ukrainian navy ship at the Crimean port of Yevpatorya.
12/12 Ukraine crisis
Crimea's pro-Moscow leader Sergei Aksyonov speaks to the media in Simferopol on the 8th March. He has defended a decision to hold a referendum on whether the region should join Russia, saying on Saturday that "no one" could cancel the voting.
The US estimates that 20,000 Russian troops are in Crimea, while Moscow says it has no forces in the region beyond those normally stationed there with its Black Sea Fleet - an assertion ridiculed by the West.
The worst face-off with Moscow since the Cold War has left the West scrambling for a response, especially since the region's pro-Russia leadership declared Crimea part of Russia last week and announced a March 16 referendum to confirm it.
US Secretary of State John Kerry, speaking to Russia's foreign minister for the fourth day in a row, told Sergei Lavrov on Saturday that Russia should exercise restraint.
“He made clear that continued military escalation and provocation in Crimea or elsewhere in Ukraine, along with steps to annex Crimea to Russia, would close any available space for diplomacy, and he urged utmost restraint,” a US official said.
Speaking by phone on Saturday, US President Barack Obama assured the ex-Soviet Baltic states which have joined NATO - Latvia, Lithuania and Estonia - that the Western military alliance would protect them if necessary. Like eastern and southern Ukraine, the three all have large ethnic Russian populations.