The United States on Monday signed a $1 billion loan guarantee for Ukraine in support of its economic recovery after weeks of political upheaval and the loss of Crimea to Russia.

It came as pro-Russian insurgents gained more ground in eastern Ukraine, occupying a police station and an administrative building, their latest seizures in a campaign that threatens to break up the ex-Soviet state.

Germany said Monday there are many signs Russia is backing pro-Kremlin militias who have seized government buildings in eastern Ukraine, including the arms, uniforms and appearance of their members.

Chancellor Angela Merkel's spokeswoman said Berlin "is worried about the further aggravation of the situation in the east of Ukraine", and that Merkel had discussed the issue with Britain's Prime Minister David Cameron.

Pro-Russian insurgents gained more ground on Monday in Ukraine's separatist east while Kiev's Western-backed leader sought a way out of the crisis by proposing a referendum and seeking U.N. help.

The Kremlin added an ominous tone to the rapid escalation by saying that President Vladimir Putin had received "a lot" of requests from Ukraine's Russified rust belt "to help, to intervene in some form".

Ukraine's interim president on Monday made a dramatic about-face aimed at defusing tensions in the separatist east by backing a national referendum on turning the ex-Soviet republic into a federation with broader regional rights.

European powers meanwhile sought to raise the pressure on Russia -- which it blames for fomenting the tensions -- with Britain calling for "further sanctions" on Moscow ahead of a meeting of EU foreign ministers.

Russian Foreign Minister Sergei Lavrov on Monday urged Washington to explain Russian media reports that CIA director John Brennan visited the Ukrainian capital.

Lavrov also accused the West of "off the scale" hypocrisy over the crisis in Ukraine, where protests in the southeast led to fresh violence at the weekend.

Ukraine's latest ultimatum to pro-Kremlin militias who have seized buildings across a swath of the separatist east expired on Monday without any sign of the gunmen ready to give in.

The streets of the impoverished coal mining town of Slavyansk remained deserted and silent despite the Western-backed interim president's vow to unleash a "full-scale anti-terrorist operation" involving the army should the gunmen not give up the local police station and state security office by 0600 GMT.

Attacks on police and security service buildings in eastern Ukraine by pro-Russian gunmen bore "tell-tale signs of Moscow's involvement," the U.S. envoy to the United Nations said Sunday.

Speaking on ABC television's "This Week" program, Ambassador Samantha Power dismissed suggestions that the attacks, which have triggered gun battles with Ukrainian special forces, were the work of grass-roots militia groups.

Russian Foreign Minister Sergei Lavrov is to meet Chinese President Xi Jinping in Beijing next week and will discuss with him the situation in Ukraine, the Russian foreign ministry said Sunday.

Lavrov will visit China on Tuesday, ahead of four-way talks between diplomats from the European Union, United States, Ukraine and Russia planned in Geneva on Thursday. Those talks are now in doubt amid violent clashes between supporters of Kiev and pro-Russian protesters in eastern Ukraine.

Ukraine's acting president accused Russia on Sunday of waging war in his country's separatist eastern rust belt and declared the launch of a "full-scale anti-terrorist operation" that left at least two dead, as Moscow said Kiev was waging a “war” against its own people.

The clashes broke out a day after masked gunmen stormed a series of police and security service buildings in coordinated raids that Washington's U.N. envoy Samantha Power said on Sunday bore "tell-tale signs of Moscow's involvement."

Ukraine on Sunday launched an "anti-terrorist operation" against pro-Russian gunmen holed up in a police station in the restive east, as Washington warned Moscow to de-escalate the crisis or face the consequences.

The offensive against the separatist militants threatens to further escalate tensions with Russia, which has 40,000 troops massed on Ukraine's eastern border and has warned Kiev against the use of force.