The department has encouraged American travellers to be cautious at holiday festivals and markets, urging them to avoid large crowds and be vigilant when visiting tourist sites.

London - Tourists were warned of a “heightened risk of terrorist attacks” at Christmas markets in Europe.

The US State Department cited “credible information” that both Islamic State and al-Qaeda militants plan to target the upcoming festive season.

It also warned that “self-radicalized extremists” may also be planning attacks

The department has encouraged American travellers to be cautious at holiday festivals and markets, urging them to avoid large crowds and be vigilant when visiting tourist sites.

It also said there could be a risk in using public transport or meeting in places of worship, hotels, restaurants and other gathering spots.

The advice praised European efforts to disrupt plots, but said the US remains concerned about the potential for attack.

The American warning came as it was revealed that a suspect held in France over an alleged Christmas terror plot had tried to enter Britain with a false passport.

French anti-terrorism teams arrested seven men, four of them in Strasbourg – a city where one of oldest and largest Christmas markets is due to open this week.

Two men were arrested in Marseille. One was said to be Moroccan refugee Hicham el Hanafi, 26, who police believe could have been plotting a “lone-wolf” attack.

French newspapers reported that el Hanafi had tried to enter the UK on a false passport, but it was unclear when or for what reason.

The other suspects are five Frenchmen and an Afghan. Sources said the group were under orders from unidentified commanders in Syria.