Latest: Ukraine 'on the brink of disaster' as Russian troop movements prompt stand-off

Acting Prime Minister calls for international help. World leaders warn Putin as Russian troops mass

An Orthodox priest holds a crucifix next to one of many armed men in military fatigues blocking access to a Ukrainian border guards base (Getty Images)

They stood together with their arms linked: a priest, a former soldier, two housewives and a teacher, among the dozen women and men in front of a Ukrainian military base the Russian troops wanted to enter and disarm.

The siege at Perevalne was an act of symbolic defiance against overwhelming might, as the Kremlin closed in on total control of Crimea. Ukraine's acting Prime Minister described the country as being on the "brink of disaster" while his government ordered the full mobilisation of its army in response to Russian military movements across the peninsula. Meanwhile the US Secretary of State John Kerry hit out at Russia's "incredible act of aggression."

Kiev had asked its forces in Crimea, numbering around 3,500 and facing up to 30,000 better-armed Russians, not to "react to provocation". It had denied suggestions that some senior officers welcomed Moscow's intervention. But just hours later, Rear Admiral Denys Berezovsky, appointed in his post as head of the navy on Saturday, defected and pledged allegiance to the new  pro-Russian administration in Crimea. The Ukrainian government announced that he would be tried for treason; but it was a humiliating blow.

The troops in Perevalne, however, did not appear to be in the mood to give up. The stand-off at the base went on all day while negotiations took place, but no shots had been fired as night fell.

Father Ivan, the parish priest of St Mary Pakrova Savatoi, the Ukrainian Orthodox church next to the camp entrance, said: "I woke up this morning to find troops of a foreign power had come with their guns. Soldiers from here come to our church: a lot of them are very young men; we will pray for their safety.

"We will pray for these Russian boys as well to protect them from harm which may be caused because of the actions of politicians, people who do not understand the terrible things which may come from what they are doing."

Ukraine's acting Prime Minister,  Arseniy Yatsenyuk, hit out at what he called "a declaration of war by Vladimir Putin", and asked for international help. The United States and Britain, along with Russia, are co-signatories of the 1994 Budapest Memorandum which guarantees the security of Ukraine against external aggression, although legal interpretation differs on whether the threat has to involve nuclear weapons.

The Russian Prime Minister, Dmitry Medvedev, said on Facebook on Sunday night that Ukraine's leaders had seized power illegally, and he predicted their rule would end with "a new revolution".

"Russia is ready to develop multi-faceted, respectful relations with brotherly Ukraine - mutually beneficial and effective relations," he said.

"But Ukraine for us is not a group of people who, pouring blood on the Maidan [Kiev's main square], seized power in violation of the constitution and other state laws."

The British Foreign Secretary, William Hague, landed in Kiev on Sunday to hold talks with the administration formed after the overthrow of Viktor Yanukovych.

During a 90-minute telephone conversation a day earlier, Barack Obama had warned Mr Putin that "continued violation of international law in regards to Ukrainian sovereignty and territorial integrity will lead to greater political and economic isolation". Mr Kerry threatened to boycott the G8 summit in Sochi, and said he had discussed an array of punitive measures, including visa bans, asset freezes and trade and investment restrictions, with allied states to "go to the hilt" in isolating Russia.

Laurent Fabius, the French Foreign Minister, said planning for the G8 summit should be put on hold and Moscow must be made to understand that the "military escalation must have costs".

But Moscow's march into Crimea - whose ethnic Russian majority population is expected to vote in a referendum for secession from Ukraine in a prelude to fulfilling the wish of many to be ruled by Moscow - has continued remorselessly. The main port, Sevastopol, and the capital, Simferopol, as well as main airports and transport and communications centres have been taken over by Russian forces.

Armed men in military fatigues stand near a painted Ukrainian coat of arms symbol as they block access to a Ukrainian border guards base (Getty Images)

The Kiev government has insisted that 10 naval ships remained armed and loyal at Sevastopol. But that was before Admiral Berezovsky appeared alongside Sergey Aksyonov, the leader of the newly established separatist Crimean administration, to say he would disregard any orders from the "self-proclaimed" government. Admiral Berezovsky had countermanded instructions to carry out a strategic withdrawal to Odessa. Two Russian warships, from the Baltic fleet, appeared offshore, in breach of bilateral agreements on military movements, as a warning to any ship captain who had decided to strike out on his own.

Meanwhile a number of military installations, including an anti-aircraft missile base, have been taken over - seemingly without significant resistance - in the last few days. But the Ukrainian forces at Perevalne were not prepared to hand over their complement of light artillery and armour, after their adversaries arrived in four armoured carriers and 13 trucks.

As had been the case hitherto, there were no insignias on the combat outfits of the force, faces were hidden behind balaclavas. Previously the vehicles the forces had been travelling in had no number plates; now some of them bore Russian ones.

A man holds a Russian flag as armed men in military fatigues block access to a Ukrainian border guards base (Getty Images)

The Ukrainians pulled up a row of tanks behind the gates to the entrance. A little later an officer, a lieutenant colonel, stepped out to hold talks with a group of Russians. On his way back to the base, he curtly said that he was "returning from negotiating with the Russian Federation".

Soon afterwards two black four-wheel-drive vehicles arrived flying flags of the Russian Unity party which has just become the governing body in Crimea. Six members of the riot police, the Berkut, who had been disbanded by the government in Kiev for their part in the killing of protesters in the capital, emerged and walked into the base.

Some among the crowd who have gathered outside were of the opinion that they had gone in to try to persuade the troops to give up. The Russian troops were professional and thorough as they secured the perimeter of the base. Those lining the route in had their Kalashnikov assault rifles loaded. Those nearer the base had unloaded the magazines into the pockets of their combat jackets.

As in other public deployments so far, they refused to answer questions, and, among themselves, the only words spoken were by non-commissioned officers repositioning some of their men.

An Autonomous Republic of Crimea and a Ukrainian flag float side by side as a man watches armed men in military fatigues (Getty Images)

"Why have you got your faces hidden, what are you ashamed off?" asked Jalil Ibrahimov, a member of the Tatar community, which is vehemently opposed to joining Russia. The two soldiers this was addressed to did not answer, but another man, who had come to support Moscow, Oleg Senkov, was affronted: "Why don't you wait until the referendum then you'll see how we really feel over here."

A debate between the two sides soon became heated and then angry when a group of pro-Russian men, burly, some in mismatched combat kit and boots, some had been drinking, appeared and started to hurl insults. One man, waving a Russian flag, kept shouting: "There is no Ukraine, only one big mother Russia." Another, who claimed to be a veteran of the Soviet Union's Afghan war, declared that those who opposed the Russians should be ashamed of themselves.

Viktor Kostenko was there to offer solidarity to the Ukrainian garrison. "I don't know whether that man served in Afghanistan. But I was in the old army and I am proud of that. But we are now Ukraine; you cannot go back to the past and I want us to be in a Ukraine for all the people who live here."