The Pakistan capital is on high alert following intelligence reports that suicide bombers have entered Islamabad in the wake of fierce clashes between security forces and militants in the restive Swat region.

Authorities in Islamabad sounded a red alert on Sunday evening after receiving information that two to three suicide bombers have entered the city, administration and police officials said.

Policemen in uniform and plainclothes have been deployed across Islamabad and new check-points were set up to
monitor the movement of vehicles.

Security agencies are maintaining a strict watch at inter-city bus stations and hotels and motels in the suburbs have been searched, officials said.

The officials described the level of the threat as "very high" and said suicide bombers could strike in retaliation for operations being conducted by paramilitary forces against pro-Taliban militants led by Maulana Fazlullah in the Swat valley and tribal rebels in North and South Waziristan.

Police officials believe security forces, government buildings and foreign missions could be "prime targets" for the suicide attackers.

"We have taken precautionary measures on such a scale that the entire city can be sealed off within minutes," a senior police official told Dawn newspaper. 

Heavy contingents of security forces, armed with metal detectors and sophisticated weapons, have been deployed at all 72 entry and exit points of Islamabad and 32 other "sensitive" locations.

Check posts have also been set up on roads connecting Islamabad to other cities to keep an eye on vehicles entering the capital.

Law enforcement agencies in the twin city of Rawalpindi have also been put on alert, officials said.

Security forces and government troops backed by helicopters have been engaged in fierce fire fights in the Swat valley.