The US, UK and Germany are on alert after the FBI warned that Islamic terror groups based in Germany may have sent parcel bombs.

The authorities at Frankfurt airport said on Friday that they had increased checks on packages destined for the US and UK.

"We take those threats very seriously," a police spokesman at the airport, Klaus Ludwig, said.

"The security standards at Frankfurt airport are already very high but we have raised them further," he said

The airport has a pressurised chamber designed to set off bombs fitted with altitude triggers.

Frankfurt police said the FBI warning had triggered a range of security measures in the city, including at post offices.

Washington and London have warned people not to open packages with unfamiliar Frankfurt markings on them.

The US Postal Inspection Service said it was screening all incoming parcels and first-class mail from Frankfurt at US airports.

The FBI warning said it had "unsubstantiated information" that one or more people "may be planning to send bombs in small parcels" to the US. A senior British police officer said the UK was also a possible target.

Fourth person sought

The increase in security came as US authorities continued to investigate possible links between Islamic terrorists and three people arrested in the last 10 days after entering the country from Canada.

Canadian police have also issued an arrest warrant for a fourth person, thought to be an associate of Ahmed Ressam, a 32-year-old Algerian who was detained in the US on 14 December.

Canadian police said they were seeking Abdelmajed Dahoumane, 32, on two charges of illegal possession of explosives.

No explosives were found in an earlier search of the apartment in Montreal that Mr Dahoumane has occupied since 1997.

Mr Ressam is accused of trying to smuggle explosives from Vancouver, British Columbia, into Washington state.

Bin Laden link

Intelligence chiefs are examining information from Canadian investigators who believe Mr Ressam underwent training in Afghanistan in the early 1990s, before moving to Canada in 1994.

A Canadian intelligence officer told the Toronto Globe and Mail newspaper that Mr Ressam had been trained in bomb-making at the Afghan base of Osama bin Laden, who is accused of past terrorist strikes on American targets.

Another Algerian national and a Canadian woman were arrested at the Canadian border on Sunday.

US officials have believed for some time that Mr Bin Laden would try to launch new attacks around the millennium and warned US citizens travelling overseas to be on their guard after the arrest of a dozen of his alleged followers in Jordan earlier this month.

The US authorities told their diplomats in Pakistan on Thursday to avoid the Northwest Frontier province, including the Khyber Pass on the border with Afghanistan.

In addition, US businesses in Belgium have advised their staff to exercise caution over the new year period after threats of attacks against US interests, the Belga news agency reported on Friday.