French police have arrested 12 people in anti-terror sweeps across southern France.

Police on Tuesday morning detained nine people with guns and ammunition near the southern port city of Marseilles and nearby Avignon.

Another three men, linked to a man of Algerian origin caught in Italy with a bomb-making kit, were arrested near Marseille and Bordeaux.

The three are suspected of supplying false papers for militants trying to return to France after training in Afghanistan.

Officials said the two security operations were not linked. The arrests come as Britain, the United States, Japan and Sweden publicly warned their citizens of possible terrorist attacks in Europe.

The U.S. State Department has warned of possible strikes by al-Qaida and its affiliates in Europe. It said terrorists could use a variety of "means and weapons" and target both official and private interests.

Britain's security agency MI5 has upped the threat level from international terrorism in Britain to severe, indicating that a terrorist attack is highly likely. The governments advised caution in public places, public transport and tourist attractions.

Germany's Interior Ministry was the exception, saying Monday there were no immediate indications of a terrorist attack against Germany.

Security officials in Europe and the United States for days have discussed the possibility of armed attackers staging a repeat in Europe of the 2008 assault against Mumbai, India that left 166 dead and scores wounded.

Western intelligence officials have said they had uncovered a plot by al-Qaida-linked militants based in Pakistan for terror attacks in major British, French and German cities.

Intelligence officials say some recent missile attacks against militants in Pakistan's northwestern tribal region were aimed at thwarting the plot.