A top US security official warned Europe on Thursday that it faced a persistent terror threat, EU ministers said, as France cited US intelligence claiming EU-born militants could strike European targets.

US Deputy Homeland Security Secretary Jane Holl Lute was invited by European interior ministers to Luxembourg to provide more details on a recent US travel alert on a potential terror strike in Europe.

But ministers said the senior US official provided no fresh details on the alert, including the name of countries that are threatened.

"Ms Lute confirmed the reality and the persistence of the threat without specifying the exact targets," European Union counter-terrorism coordinator Gilles de Kerchove told a news conference following the meeting.

French Interior Minister Brice Hortefeux confirmed that US authorities informed France that a group of militants from EU countries was planning to return to Europe after combat training in the Afghan-Pakistani border region.

"The threat is real," Hortefeux told reporters.

"This is a certain number of European citizens trained in Afghanistan and Pakistan and who are likely to return to Europe with bad intentions," he said.

The US official did not speak to the media during her visit.

Since the United States issued its alert on Sunday, Britain, Japan, Sweden and France have issued their own travel warnings.

Europeans wanted to get more details on the US travel alert, which warned of a potential terror strike, notably on transport systems and tourist attractions.

Holl Lute was short on details during the discussions, ministers said.

"It was nothing specific, nothing very new," said Swedish Justice Minister Beatrice Ask.

German Interior Minister Thomas de Maiziere said: "We agree that there is no indication of concrete targets, concrete dates and concrete terror groups."

But ministers said the senior US official indicated that one of the threats could be a "multi-target" strike -- the type of tactic used in the Mumbai commando-style attack in 2008.

In that assault, 10 militants from the Al-Qaeda-linked group Lashkar-e-Taiba mowed down unarmed civilians at targets across the Indian city after arriving by boat from Karachi, leaving 166 dead.

A French intelligence official told AFP on Wednesday that "25 young people trained in combat" in the lawless Afghan-Pakistani border region were getting ready to return home to Europe.

The source said US intelligence services alerted their counterparts in Europe to the group's activities "a few weeks" ago and the warning was reiterated by British authorities.

One French official said the group has experience of fighting in the lawless border region where US-led forces are trying to root out leaders of the Al-Qaeda network, and could be able to organise other extremists in Europe.

De Kerchove said what is worrisome about militants who hold EU passports is that "they are hard to detect."

"They have European passports and they are not known by police. They left to train and are ready to act," de Kerchove said.