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Ukraine's armed forces took what may prove to be one of their final stands Saturday in Crimea, as pro-Russian forces stormed and seized control of an air force base amid a barrage of gunfire and explosions.

A tense blockade of the Belbek air base base that has endured for more than a week looked set for an inevitable culmination following the seizure of one Ukrainian-held military facility after another in recent days.

It was the last major Ukrainian military facility in Crimea to fall into the hands of pro-Russian forces. The Ukrainian Defence Ministry hasn't provided details of how many bases it still controls on the peninsula.

Crimea residents voted last week to secede from Ukraine and join Russia -- a process that was formalized this week with the blessing of President Vladimir Putin. The vote, which was held under condition akin to martial law under the gaze of apparently Moscow-led militia forces, has been rejected as illegitimate by the international community.

The assault on the Belbek base mirrored events at other Ukrainian-held military facilities on the peninsula in recent days.

In footage provided by the Ukrainian Defence ministry, a Russian-made BTR-80 armoured personnel carrier could be seen smashing open a front gate at Belbek, a base across the bay from the port city of Sevastopol.

APCs crashed through walls at two other locations and were followed by armed personnel, who advanced in crouching position as they secured the area. Four BTR-80s were involved in the assault, Ukrainian officials said.

Ukrainian troops offered no resistance. Later, a separate motley group arrived at the scene. The crowd appeared to be made up of professional soldiers, members of a recently-formed militia unit and Cossacks.

The cause of the explosions wasn't immediately clear, although Ukrainian officials said they were stun grenades used to disperse any potential resistance.

Two ambulances arrived and then departed shortly after. Ukraine's Defence Ministry said one reporter and a Ukrainian soldier were injured in the raid.

After the takeover, Belbek base commander Col. Yuliy Mamchur called together his men, who sang the Ukrainian national anthem and then stood at ease. He then told his men to put their weapons in the base's armoury.

A few hours before, Mamchur attended a wedding between two lieutenants serving at Belbek. Soldiers drank champagne and toasted the couple, despite the looming threat of a raid on the base.

Earlier, a crowd stormed the Novofedorivka base, about 50 kilometres west of Crimea's capital, Simferopol, Ukraine's Defence Ministry said.

Ukrainian television station TSN said troops inside the base hurled smoke grenades in an attempt to disperse groups of burly young men attempting to break through the front gates.

There were conflicting reports about whether the base was eventually taken over.

The Russian Defence Ministry says that as of late Friday, less than 2,000 of 18,000 Ukrainian servicemen in Crimea had "expressed a desire to leave for Ukraine." The ministry, however, stopped short of saying the remainder of the troops would serve in the Russian army.

No similar information has been forthcoming from Ukraine's authorities, who have been criticized by servicemen marooned in Crimea, some of whom have complained to media that they have been given no clear instructions on what they should be doing.

Elsewhere, more than 5,000 pro-Russia residents of a major city in Ukraine's east demonstrated in favour of holding a referendum similar to the one carried out in Crimea.

The apparent ease with which Russia has managed to take control over Crimea has ignited concerns for Ukraine's government that elements in the mainly Russian-speaking east will agitate for a similar move.

Russia has brought large military contingents to areas near the border with eastern Ukraine. Putin has said there is no intention to move into eastern Ukraine, but the prospect of violence between pro- and anti-secession groups in the east could be used as a pretext for sending in troops.

Valentina Matvienko, the speaker of the upper house of the Russian parliament and a close Putin ally, said Saturday there is no intention to absorb other regions of Ukraine, according to the ITAR-Tass news agency.

Eastern Ukraine is the heartland of Ukraine's economically vital heavy and mining industries. It's also the support base for Viktor Yanukovych, the Ukrainian president who fled to Russia last month after three months of protests in the capital, Kyiv, triggered by his decision not to sign an agreement with the European Union.

Russia and Yanukovych supporters contend the former leader's ouster was a coup and allege that the authorities who then came to power are nationalists who would oppress the east's large ethnic Russian population.

"They're trying to tear us away from Russia," said demonstrator Igor Shapoval, a 59-year-old businessman. "But Donbass is ready to fight against this band which already lost Crimea and is losing in the east."

Donbass is the name for the region of factories and mines that includes Donetsk.

About an hour after the Donetsk rally began, the crowd marched through the city centre and assembled before the regional administration building chanting: "Crimea! Donbass! Russia!"

Demonstrators waving Russian flags were faced off by lines of riot police. Inside, German Foreign Minister Frank-Walter Steinmeier met with local officials.

"We heard that there is much desire to preserve Ukraine as a single state," he told reporters after the meeting.

The demonstrators erected several tents -- an ironic echo of the huge tent camp that was established on Kyiv's central square after the protests against Yanukovych broke out in late November.

"I'm ready to live in a tent, but I'm not ready to submit to the West, to dance to their tune," said Viktor Rudko, a 43-year-old miner.

The rally in Donetsk in the end dispersed without any disturbances. Another similar meeting is expected Sunday.

The local provincial parliament on Friday formed a working group to develop a referendum analogous to the one in Crimea. Activists on Saturday passed out mock ballots, although no referendum has been formally called.

A number of leading pro-Russian activists have already been detained by police on suspicion of fomenting secessionist activities. The country's security services said Saturday that they have arrested Mikhail Chumachenko, leader of the self-styled Donbass People's Militia, on suspicion of seeking to seize authority.

U.N. Secretary-General Ban Ki-moon met with Ukraine's acting Prime Minister Arseniy Yatsenyuk in Kyiv on Saturday and said: "I am confident that with such strong support from the international community that you are receiving, under your leadership as well as with your courageous people, you will be able to overcome this difficult time."

Sanctions imposed this week by the U.S. and the European Union haven't persuaded Russia to back off on its intent to annex Crimea.

As tensions roil in the east, the Organization for Security and Cooperation in Europe is deploying an observer team aimed at easing the crisis.

Russian Foreign Ministry spokesman Alexander Lukashevich said in a statement on Friday that Moscow hopes that the 200-strong team "will help to overcome the internal Ukrainian crisis" and ensure the respect for human rights there.

It is unclear whether the team will be allowed into Crimea. Russian forces last week stopped OSCE military observers from entering Crimea. The organization on Friday didn't specify whether the observers will go to Crimea.

Lukashevich said Saturday that the OSCE's mission "will reflect the new political and legal order and will not cover Crimea and Sevastopol, which became part of Russia."

Sevastopol, a city in southwest Crimea, is the home of Russia's Black Sea Fleet.

Daniel Baer, the chief U.S. envoy to OSCE, said the observers should have access to the territory because Crimea remains Ukrainian to the rest of the world.