'Gas war'
 
The European commission has called a special meeting of its gas co-ordination group for March 11 "to ensure a fully coordinated EU response to the situation".
 
The row echoed a 2006 dispute when Gazprom cut off all gas to Ukraine because Kiev had refused to agree to a much higher price for supplies, triggering a ripple effect of shortages across Europe.
 
Viktor Yushchenko, Ukraine's president, warned earlier that the dispute could lead to a "gas war" with Moscow.
 
The row is the first test for Dmitry Medvedev, Russia's president-elect, who has previously defended the country's tough negotiating stance with former Soviet republics such as Ukraine.
 
Medvedev spoke to Yushchenko by telephone on Tuesday, urging "an intensification of efforts by Kiev" to resolve the dispute "as soon as possible".
 
Gazprom insisted supplies to European Union customers via Ukraine would "be assured at full volume".
 
But in a statement, Naftogaz said: "Naftogaz can only guarantee uninterrupted transit (of gas) to European consumers as long as it does not threaten Ukraine's energy security.
 
"If Gazprom continues to brutally violate the technical agreements between the two countries... Naftogaz will reserve the right to carry out adequate and asymmetrical measures to protect its consumers."
 
Political background
 
Volodymyr Ogryzko, Ukraine's foreign minister, promised during a visit to Warsaw that his government would "do everything" to ensure that gas deliveries to the European Union continued "normally".
 
Gazprom placed the blame for the cut firmly on Ukraine, saying Kiev had failed to pay back a $600m debt following the initial 25 per cent supply cut on Monday.
 
Naftogaz has sounded a defiant note, saying the country could survive for another month by drawing on its own natural gas production and some reserves.
 
However, the company said it was still open to negotiations.
 
Ukraine has accused Russia of using the gas question to punish Kiev's pro-Western leadership, which has sought to wrest Ukraine from Moscow's sphere of influence and towards greater integration with Europe.
 
Yushchenko and Vladimir Putin, Russia's president, had announced last  month that they had reached a preliminary agreement on regulating gas supplies between the two countries, but the final deal has not been signed.