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 yesterday 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 Organisation 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 trying 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 defuse a crisis that threatens to splinter Ukraine, an ex-Soviet state of 46 million people perched between Russia and the EU.
Obama spoke to German Chancellor Angela Merkel 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.
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 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 the area to Russian rule.
And in Donetsk, about 400km 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 on Thursday.
Riot police encircled the building before last night's planned demonstration by pro-Russian activists.
- AAP
