In what could lead to a radical re-examination of NATO's defense doctrine, a leading U.S. senator has called on the alliance to come to the aid of any member whose energy sources are threatened by using the organization's Article 5 mutual defense clause.

Senator Richard Lugar, the chairman of the Senate Foreign Relations Committee, told a gathering of security experts here that any NATO member whose energy sources are cut off by force should be able to rely on assistance from the alliance.

The proposal has received a cautious welcome from the countries of Eastern Europe, which are the most vulnerable to energy shortages because of their heavy dependence on Russia.

"Article 5 of the NATO charter identified an attack on one member as an attack on all. It was also designed to prevent coercion of a NATO member by a non-member state," Lugar said in an address to a conference organized by the German Marshall Fund of the United States.

"We should recognize that there is little ultimate difference between a member being forced to submit to coercion because of an energy cutoff and a member facing a military blockade or other military demonstration on its borders," Lugar said.

Poland had already taken the lead in the 26-member alliance in calling on NATO to play a greater role in energy security.

Following Russia's decision last January to cut its gas deliveries to Ukraine over a price dispute and then an accord between Germany and Russia to build a gas pipeline under the Baltic Sea that will bypass Poland, Warsaw has tried to rally support from NATO and the European Union to protect countries dependent on Russia for their energy.

Lithuania has also faced shortages since Russia stopped sending oil to its refinery after an oil pipeline was damaged several months ago. Instead of appealing to NATO, Lithuania has asked the EU to raise the issue with the Russian president, Vladimir Putin.

Lugar said that NATO must determine what steps it would take "if Poland, Latvia or another member state is threatened as Ukraine was." He added that because "an attack using energy as a weapon can devastate a nation's economy and yield hundreds or even thousands of casualties, the alliance must avow that defending against such attacks is an Article 5 commitment."

Some East European countries are concerned that any new interpretation of Article 5 could actually weaken it rather than strengthen it.

"When it comes to energy security, there should be solidarity in the European context," said Marek Prawda, Poland's ambassador to Germany.

"Mutual assistance is necessary but I would be careful at linking it directly to Article 5."

Poland's conservative government, a staunch supporter of the alliance, is already concerned that NATO could be weakened if it established closer ties with Australia, New Zealand and Japan - countries that have been involved in NATO-led peacekeeping missions.

Czech security experts agreed with Lugar that NATO should play a greater role in energy security, but they were skeptical about its ability to provide assistance.

"I would like NATO to deal with energy issues but not to deal with it with tanks. If you open up the legal meaning of Article 5, we may lose more than we gain," said Jiri Schneider, director of the Program of Atlantic Security Studies in Prague. "The question is if NATO can provide something. Has it the capacity to develop energy pipelines or organize airlifts for energy?"

Lugar said NATO could respond in several ways. "It should develop a strategy that includes the re-supply of a victim of an aggressive energy suspension," he said. "Alternatives to existing pipeline routes must be identified and financial and political support for the development of alternative energy sources is crucial." Lugar has made energy security one of his main foreign policy concerns.

Peter Balazs, an economics professor and director of EU strategy at the Central European University in Budapest, said Article 5 was about mutual defense. "It means dangers and conflict. The EU would be a more appropriate institution to deal with this issue," Balazs said. West European governments have tended to look to the EU rather than NATO in dealing with the issue of energy security.