Speaking to reporters in Washington, Mr Cohen said that force should remain an option to prevent oil reaching Belgrade.

Many countries outside Nato and the EU are joining the embargo. But Russia in particular has failed to agree to adhere to an embargo.

"There will be serious consequences that would flow to countries that would openly and flagrantly supply energy to a regime that has engaged in this kind of brutal behaviour," Mr Cohen said.

The BBC's Paul Reynolds in Washington: "Embargo will have a widespread effect"

"I would point out that any third country that appears to be in violation of the European Union position or that of Nato certainly runs the risk of economic or other types of political reaction as well as the potentiality of military."

Mr Cohen said he hoped that Russia would not try to run any blockade.

Stop-and-search plan

Nato is still working out how to enforce the ban.

Nato's supreme commander General Wesley Clark has requested authority for ships suspected of carrying oil to Yugoslavia to be stopped and searched.

"I believe he ... should have whatever flexibility is required for him to achieve his goals," Mr Cohen said.

In some European countries, questions have been raised about the legality the plan.

France says this would require a United Nations mandate, warning that searching and seizing ships by force would be "an act of war" that could bring the alliance in conflict with Russia.

BBC Washington Correspondent Paul Reynolds says it is likely that Nato, which needs a consensus, will mainly tackle the problem at source by trying to persuade countries and companies not to load or ship oil.