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.
The latest show of pro-Moscow sentiment in the largely Russian-speaking southeast comes after Russia threatened Friday to halt gas supplies to Ukraine following Western sanctions to punish the Kremlin for seizing de facto control of Crimea.
The warning by Russian state-run energy giant Gazprom, which could affect supplies to other countries, raised the spectre of previous gas disputes between Russia and Ukraine that deeply rattled European economies in 2005-2006 and again in 2009.
Gazprom said the move was in response to unpaid bills, but the threat -- made after the European Union warned it could toughen sanctions against Moscow -- underscored the Kremlin's resolve to stand its ground in the biggest East-West crisis since the Cold War.
In a sign of the tensions racking Crimea, Ukraine's defence ministry said late Friday that unidentified militants had smashed through the gates of a Ukrainian air force base in Sevastopol.
No shots were fired in the incident.
A convoy of foreign observers from the Organization for Security and Co-operation in Europe (OSCE) was earlier stopped at a checkpoint in Crimea guarded by pro-Kremlin gunmen.
Russia's foreign ministry accused the OSCE of attempting to enter the Black Sea peninsula uninvited and "without considering the opinions and recommendations of the Russian side".
The OSCE observer mission is a crucial part of the "off-ramp" US President Barack Obama is pushing to de-escalate a crisis that threatens to splinter Ukraine, an ex-Soviet state of 46 million people perched on the threshold between Russia and the EU.
Obama spoke with German Chancellor Angela Merkel on Friday and hailed the US and EU's "unified position" on Ukraine, the White House said.
Obama and Merkel "agreed on the need for Russia to pull back its forces" and allow international observers and human rights monitors into Crimea, the statement said.
It said the leaders had also discussed a "contact group" to lead a direct dialogue between Russia and Ukraine -- an idea the Kremlin has scorned.
In Moscow, police said more than 65,000 people had attended a rally outside the Kremlin supporting Russia's full annexation of Crimea, a predominantly ethnic-Russian peninsula roughly the size of Belgium.
The heads of Russia's two houses of parliament said they would respect the decision by the flashpoint region's parliament to split from Kiev and hold a March 16 referendum on switching to Russian rule.
And in Donetsk, about 400 kilometres (250 miles) northeast of Crimea, tensions were high around Lenin Square, where pro-Russian activists have set up a round-the-clock picket under a red Soviet flag.
Militants occupied the city's regional government offices for three days this week, hoisting a Russian flag before being dislodged by police Thursday.
Riot police encircled the building ahead of Saturday's planned demonstration by pro-Russian activists.
Rival demonstrations brought thousands of people into the streets this week and degenerated into running street battles on Wednesday.
The EU has vowed to sign a landmark trade pact aimed at pulling Kiev out of Moscow's orbit before Ukraine holds snap presidential polls on 25 May.
Yanukovych's decision to ditch that pact in November in favour of closer ties with Russia sparked the initial wave of protests that led to his regime's downfall late last month, and the rise of Ukraine's new pro-EU government.
With Russian forces in effective control of Crimea -- a region of two million people and the base of the Kremlin's Black Sea Fleet -- the threat of Ukraine splintering seemed more real than at any point since Putin won parliamentary approval to use force against his western neighbour.
Western allies have been grappling with a response to Putin's seeming ambition to create a Soviet-style sphere of influence that Moscow argues provides a defence for ethnic Russians coming under attack.
"Can Russia stand idly by when Russians somewhere in the world -- especially in neighbouring Ukraine -- face mortal danger?" Putin spokesman Dmitry Peskov asked on Russian state television Friday.
Russian Foreign Minister Sergei Lavrov meanwhile told US Secretary of State John Kerry that Washington's sanctions against Moscow -- which so far include visa bans and asset freezes on targeted individuals -- would "boomerang" back on the United States.
The foreign ministry added that Russia would not leave any EU punitive measures "without a response".
Gazprom -- often seen as a political weapon wielded by the Kremlin against Western-leaning ex-Soviet states -- said a few hours later it might have to cut off Ukraine for the first time since 2009 due to a debt of $1.89 billion (1.36 billion euros).