Armed men, thought to be Russians, seize a Ukrainian military base in the Crimean port city of Sevastopol. Meanwhile, Ukraine’s premier rejects Crimea’s planned referendum.
A Reuters journalist said no shots were fired in the incident. Negotiations between the two sides are said to be continuing.
Initial reports said a truck had smashed through the gates and that a post in Sevastopol had been stormed.
One Ukrainian military officer, Vladislav Seleznyov, said by phone that the armed men had taken over the base without any shooting and that no-one had been hurt.
Another Ukrainian official told Reuters that he was now mediating between the Ukrainian forces and the armed group inside, and that no arms had been seized.
Earlier on Friday, Ukraine’s interim prime minister, Arseniy Yatsenyuk, said the “so-called referendum” on Crimea joining Russia would not be accepted by anyone in the civilised world.
Talking to CNN Mr Yatsenyuk said: “No-one will recognise this referendum, apart from maybe North Korea, Syria and Venezuela.
“I want to be very clear – Crimea was, is, and will be an integral part of Ukraine. No concessions. Full stop.”
Mr Yatsenyuk said Ukraine was ready for talks with Russia, but Moscow must first withdraw its troops, abide by international agreements and halt its support for “separatists and terrorists”.
On Thursday, a total of 78 out of 86 lawmakers that make up the local Crimea parliament voted for the crisis-hit region to become part of Russia.
They set a referendum for 16 March, which will ask the Crimean people whether they would like to remain a part of Ukraine or join Russia.
His comments came after Russian President Vladimir Putin’s spokesperson Dmitry Peskov said: “Mr Putin’s efforts to de-escalate the crisis in Ukraine are met with no understanding.
“Regardless of all the efforts of our president, his readiness to explain Russia’s position practically on a daily basis, we still hit a wall of no understanding.
“It is rather sad, and what is worse is that it is very bad from the point of view of possible repercussions.”
He added Moscow was not orchestrating events in Ukraine. “Quite the opposite,” he said.
Read more: British EU chief hears Ukraine shooting claim in bugged call
Read more: how long has Russia been planning its Crimea intervention?
Read more: how the far-right took top posts in Ukraine's power vacuum
Read more: the five Sochi pictures that define the current crisis in Ukraine
Earlier on Friday Mr Putin spoke on the phone to US President Barack Obama for the second time in six days, but the pair remained far apart over Ukraine.
Mr Putin said in a statement that Moscow and Washington were still far apart on the situation in the former Soviet republic, where he said the new authorities had taken “absolutely illegitimate decisions on the eastern, south eastern and Crimea regions.
“Russia cannot ignore calls for help and it acts accordingly, in full compliance with international law,” Mr Putin said.
Before calling Mr Putin, President Obama announced the first sanctions against Russia since the start of the crisis, ordering visa bans and asset freezes against so far unidentified persons deemed responsible for threatening Ukraine’s sovereignty.
Mr Obama said he urged President Putin to accept the terms of a potential diplomatic solution, and said the dispute over Crimea could be resolved in a way that took account of Russia’s legitimate interests in the region
Mr Putin was defiant on Ukraine, where he said pro-Russian President Viktor Yanukovych had been overthrown in an “anti-constitutional coup” last month.
But he stressed what he called “the paramount importance of Russian-American relations to ensure stability and security in the world”, the Kremlin said.
“These relations should not be sacrificed for individual differences, albeit very important ones, over international problems,” Mr Putin said.
He maintained Moscow was not behind the seizure of Crimea, home of Russia’s Black Sea fleet. Russia says the troops without national insignia that have surround Ukrainian bases are “local self-defence units”.
Meanwhile, Ukraine’s Paralympic team confirmed it would take part in the Sochi Winter Paralympics, which begin on Friday.
The head of the team Valeriy Sushkevich said Ukraine would participate in the opening ceremony but would pull out of the games if Russian troops invaded Ukraine.
“I declare: we will pull out of the 11th Paralympic Games in Russia that very second if the thing we fear and which we are against… happens,” he told a news conference.
However, according to Russian Interfax news agency, Mr Putin later told Mr Sushkevich that politics and international affairs must not affect the games.
The pair discussed “the fact that a sports celebration – all the more so one like the Paralympic Games – cannot and must not be under the influence of … the international agenda or politics,” Interfax quoted Kremlin spokesman Mr Peskov as saying.