Ukraine braced Saturday for new pro-Russian protests in the tense eastern city of Donetsk after Moscow threatened to stop crucial gas supplies to the country, further escalating hostilities with the West.

Donetsk, a focal point of the crisis engulfing Ukraine since the protests that toppled president Viktor Yanukovych, was expecting a large demonstration by activists demanding a secession referendum like the one planned for the Crimean peninsula.

Accused of failing to anticipate Russian's actions in Ukraine, U.S. spy agencies insisted Friday they were not caught off-guard and had warned the White House intervention was "imminent."

The head of the Defense Intelligence Agency, Lieutenant General Michael Flynn, said the intelligence community predicted the movement of Russian troops about a week in advance.

The European Union needs to send observers to Ukraine as soon as possible, foreign ministers from the bloc's 10 eastern European and Nordic members plus Iceland and Norway said Friday.

"To help Ukraine in this period of transition, we support the establishment of a European Union led Observation Mission, which should be deployed as soon as possible," they said in a joint statement following ministerial talks in Estonia's eastern city of Narva.

France said Friday there could be no referendum on the future of Crimea unless Ukraine decided to organize one.

"The territorial integrity and sovereignty of Ukraine are non-negotiable," President Francois Hollande said after a meeting with Ukraine's former foreign minister Petro Poroshenko and Vitali Klitschko, the ex-boxer and leading figure of the protest movement that ousted the country's pro-Moscow president.

Ukraine's far-right Pravy Sektor movement on Friday announced its leader Dmytro Yarosh would make a presidential bid in elections scheduled for May 25 and said it was ready to go to war with Russia.

The ultra-nationalist group played a crucial role in the frontlines of deadly protests that unseated former President Viktor Yanukovych last month and has been branded a neo-Nazi organization by Russia.

A United States warship crossed Turkey's Bosphorus Strait Friday, headed towards the Black Sea, as tensions simmer over Ukraine's Crimea region.

A coastguard boat was seen escorting the guided-missile destroyer, the USS Truxtun, an Agence France Presse photographer saw.

Pro-Russian militants smashed through the gates of a Ukrainian air force base in Crimea on Friday in the latest signs of boiling tensions on the rugged peninsula, Ukraine's defense ministry said.

The militants entered the base and were in negotiations with the commander of the base, while soldiers guarding it have locked themselves in their barracks, a ministry spokesperson told Agence France Presse.

Interpol said Friday it was considering a request from Ukraine's new government to issue an arrest warrant for ousted president Viktor Yanukovych.

The international police organization confirmed it had received a request from authorities in Kiev on Wednesday to issue a "Red Notice", or international wanted persons alert, for Yanukovych on charges including abuse of power and murder.

The House of Representatives on Thursday overwhelmingly approved U.S. financial aid to crisis-plagued Ukraine, but the Senate is unlikely to take up the measure until at least next week.

The lower chamber of Congress voted 385 to 23 in favor of the loan guarantees, the first tangible congressional response to Russia's incursion into its neighbor and former Soviet satellite.

The U.N. Security Council on Thursday launched another round of closed-door talks on the situation in Ukraine, its fourth such meeting in less than a week.

The meeting, convened at Britain's request, was expected to discuss the mission of U.N. special envoy to Crimea Robert Serry, who was forced to cut short his trip to the region when he was confronted by unidentified gunmen on Wednesday.