A woman walks past a closed subway station outside EU headquarters in Brussels on Nov 23, 2015. [Photo/IC]

WASHINGTON/PARIS - Citing "increased terrorist threats" from militant groups in various regions of the world, the US State Department issued a global travel alert on Monday following deadly militant attacks in France and Mali.

As millions of Americans prepare to travel for the US Thanksgiving holiday on Thursday, the agency said potential attackers could target private or government interests.

"Current information suggests that (Islamic State),al-Qaida, Boko Haram, and other terrorist groups continue to plan terrorist attacks in multiple regions," the State Department said in a warning posted on its website.

Although it did not mention the Nov 13 Paris attacks claimed by the Islamic State group in which 130 died, the department noted that militants had carried out attacks in France, Nigeria, Denmark, Turkey, and Mali during the past year.

"Authorities believe the likelihood of terror attacks will continue as members of (Islamic State) return from Syria and Iraq," it said. "Additionally, there is a continuing threat from unaffiliated persons planning attacks inspired by major terrorist organizations but conducted on an individual basis.

France and Belgium have launched a manhunt following the attacks in Paris, with a focus on Brussels barkeeper Salah Abdeslam, 26, who returned to the city from Paris hours after the attacks and is still at large.

Abdeslam's mobile phone was detected after the attacks in the 18th district in the north of Paris, near an abandoned car that he had rented, and then later in Chatillon in the south, a source close to the investigation said.

Detectives were examining what appeared to be an explosive belt found in a litter bin in the town of Montrouge, south of the capital and not far from Chatillon.

The source said it was too soon to say whether the belt had been in contact with Abdeslam, whose elder brother blew himself during the gun and suicide bomb attacks.

One theory was that Abdeslam had intended to blow himself up in the 18th district but had abandoned the plan, although it was not clear why.

"Maybe he had a technical problem with his explosive belt,"a police source said.

Fearing an imminent threat of a Paris-style attack, Belgium extended a maximum security alert in Brussels for a week but said the metro system and schools could reopen on Wednesday.

"We are still confronted with the threat we were facing yesterday," Prime Minister Charles Michel said. Potential targets remained shopping areas and public transport.

Belgium has been at the heart of investigations into the Paris attacks since French law enforcement bodies said two of the suicide bombers had lived there. Three people have been charged in Belgium with terrorist offences, including two who travelled back with Abdeslam from Brussels.