Russian special forces have used armoured vehicles to storm an air force base in Crimea after Ukrainian troops refused to abandon their posts.
One Ukrainian soldier was reported to have been shot during the assault, launched after a deadline to hand over the compound to Russia expired at 12.30 GMT.
"They came through the walls in armoured personnel carriers," said Sky News Chief Correspondent Stuart Ramsay, at the Belbek base.
"There have been explosions, blast bombs, one Ukrainian soldier has been shot."
He added: "We believe they are special forces. They are all balaclaved and have slightly different uniforms to regular Russian soldiers.
"There was a lot of firing, probably into the air, it is hard to know. I've seen at least one injured soldier."
Ramsay also heard "big explosions" which he said were probably blast bombs to disorientate the Ukrainian troops, who were then made to line up on a parade ground.
The Ukrainian commander at the Belbek base had expected the attack and told his forces to resist, instructing them they could fire warning shots before fighting back.
He has since been taken into custody by the Russians. A live camera shot of the base was also taken out after a Russian soldier climbed a mast to disable it.
Earlier the Ukrainians loaded an "air defence weapon" and troops were ordered to stand their ground and not surrender the base.
However, any resistance appears to have had little effect.
Stuart Ramsay said that the Ukrainians were "massively outnumbered and outgunned" by the Russians, with just small arms and a few machine guns.
Russian soldiers had earlier surrounded the airbase while militia wearing balaclavas and Cossack-style hats gathered outside the base.
Sky's Foreign Affairs Editor Sam Kiley said the base was an important capture for the Russians.
"It is a base that is home to a significant number, possibly a third, of the main combat aircraft of the Ukrainian air force - the MIG-29s - and their support aircraft and the structures that go with them.
"If you look at that and the blockage of the Ukrainian Navy in the shared port of Sevastopol in Crimea, what you see here is the Russians doing two things.
"The first is to seize territory that they now lay claim to and the second is to cripple the Ukrainian armed forces.
"That is extremely important to them if they want to move into the Ukrainian eastern provinces where there are a predominance of Russian speakers."
The Foreign Office has extended its travel warnings and is advising against all but essential travel to Kharkiv, Donetsk and Lugansk due to increased tension in eastern Ukraine.
At the Kremlin on Friday, Russian President Vladimir Putin signed legislation incorporating Crimea into Russia and hailed it as a "remarkable event".
International monitors are due to arrive in Ukraine in the next 24 hours to examine the political and security situation in the country.
Russia agreed the move - claiming it would help stop "nationalist bandits" in the country - but has barred the observers from going to Crimea.
It also hit back at the widening of sanctions, calling them "divorced from reality" and said it reserved the right to impose sanctions of its own.
The 57 member countries of the Organisation for Security and Co-operation in Europe (OSCE) agreed an initial deployment of 100 monitors to regions in the east, south and west of Ukraine.
They will spend six months in the country and 400 more could be added "as necessary and according to the situation", diplomats said.
Western countries have been pushing hard for an observer mission as a way of preventing an escalation of tensions in Ukraine following Russia's annexation of the Crimean peninsula.
Russia had blocked the plan on previous occasions.
OSCE vice-chairperson Thomas Greminger welcomed the decision as a "very meaningful contribution to de-escalate the situation in Ukraine".
But, in a statement on Saturday, Russia's Foreign Ministry made it clear Crimea is a 'no-go area' for the observers.
It said: "The mission's mandate reflects the new political and legal realities and does not apply to Crimea and Sevastopol, which became a part of Russia."
"Russia hopes that the objective and impartial work of the international observers will help to overcome the internal Ukrainian crisis, stop rampant nationalist banditry, eradicate ultra-radical tendencies."
Prime Minister David Cameron and other EU leaders have also imposed travel bans and asset freezes on 12 more people to punish Moscow for its takeover of the Ukrainian territory.
There are now 33 Ukrainians and Russians on the list, accused of playing some part in what Western powers say is an illegal land grab of the region.
Russia said the move was necessary to prevent oppression of Russian-speaking people on the peninsula following the Ukrainian political crisis which saw President Viktor Yanukovych flee the country.