All major airports in Pakistan were placed on red alert on Thursday after a telephone caller warned of a suicide bomb attack as the country continued to be on edge with fresh threats from militants to launch more strikes on US targets.

Passengers were evacuated from the capital's Benazir Bhutto airport and the whole complex was searched as an anonymous caller threatened of a possible suicide bombing. But all flights operated as normal, officials said.

The threats of new attacks came just five days after the deadly Marriott Hotel bombing, prompting a red alert to be sounded. Simultaneously, enhanced safety measures were also taken at all airports across Pakistan.

"We have raised the security level to red alert. It was already on high alert, but after bomb threat we stepped it up," Col Ashraf Faiz, Chief Security Officer, said.

The security at the entrance and the exit gates was stepped up and no visitors were allowed inside the airport.

Coinciding with the threat to the airport, a terrorist group that claimed responsibility for the deadly suicide attack on the Marriott Hotel has issued fresh warnings that it will carry out more strikes on American targets.

Warning issued

An SMS sent to journalists in Islamabad late on Wednesday night, said the Fidayeen-e-Islam would carry out more attacks on US targets.

The group rejected the Pentagon's statement that only two US Marines were killed in the bombing of the Marriott.

"In fact three top floors of Marriott were full of Marines, NATO high-ranking officials, European diplomats and FBI agents. Secondly, all those who will facilitate Americans and NATO crusaders like (Marriott owner Sadruddin) Hashwani, they will keep on receiving the blows," it said in a message in English.

The hitherto unheard of Fidayeen-e-Islam had on Monday claimed responsibility for the September 20 bombing of the Marriott that killed over 50 people, including at least five foreigners, and injured 266.

Little or nothing is known about the group and Pakistani officials continue to suspect the hand of al-Qaeda or Taliban in the gruesome bombing.

Concerned about the possibility of further attacks, the US has prohibited all its government personnel stationed in Pakistan from staying in or even visiting major hotels in the capital and in the cities of Karachi and Peshawar.

The personnel have been told to stay away from restaurants.

The US embassy in Pakistan has also issued travel advisory precautioning all American citizens working or staying in Pakistan to follow the same directions.

The embassy also temporarily suspended issuing of visas and other consular services. However, emergency assistance to US citizens would continue.