The United States said Monday it would not force the issue of placing armed sky marshals (search) on trans-Atlantic planes if European countries reject such anti-terrorist precautions.

Asa Hutchinson (search), U.S. undersecretary for border and transportation security, told EU officials the U.S. administration would look at all other safety precautions in case of a security threat, and would move to the cancellation of a flight as a last resort.

"From our perspective, sky marshals add a deterrent factor and a safety factor that we will always consider, but we will consider other measures as well," Hutchinson told reporters after an EU-U.S. meeting to discuss cooperation.

Portugal, Sweden (search) and Denmark have signaled they prefer canceling flights to deploying armed guards on planes if there is a strong suspicion of an attack.

Hutchinson defended the use of such guards on planes, arguing they are needed to thwart terrorist attacks. But he said Washington was looking for cooperation as much as possible.

"We recognize that is not a security measure that is acceptable in all European countries," Hutchinson said. "We obviously would not make that demand" if armed marshals are not allowed under a particular country's laws, "or they don't have the resources to do it."

Together with European Union negotiators, Hutchinson agreed that "canceling any flight is a matter of last resort."

Both sides also agreed to reassess rail security in the wake of March 11 train bomb attacks that killed 191 people and injured more than 2,000 in Madrid.

"The technology that we currently have that is so applicable to aviation security is not necessarily the technology that is needed in rail and transit systems," he said.

The meeting was the first of a series of sessions between EU and U.S. officials to better coordinate anti-terrorist policies.

Both sides said there were also plans to start an international data base for missing and stolen passports, which allow terrorists to travel and also to raise money on the black market.

"We have set today a new dialogue in motion that will help us address some of the fundamental threats to our security in the months and years to come," said the EU's newly appointed terrorism czar, Gijs de Vries.