Pakistan placed all airports on high alert Thursday after a telephone caller warned of a suicide bomb threat to Islamabad's international airport, officials said, days after a huge blast in the city.

Separately Pakistani troops fired warning shots at NATO helicopters near the Afghan border, underscoring tensions between Islamabad and Washington over US military incursions. NATO said the choppers were in Afghan air space.

Passengers were briefly evacuated from the capital's Benazir Bhutto airport while security officials searched the area, but all flights were operating as normal, they said.

The security boost comes after a massive suicide truck bombing at Islamabad's Marriott Hotel on Saturday left 60 people dead and more than 260 wounded.

"We have raised the security level to red alert. It was already on high alert but after the bomb threat in Islamabad we stepped it up," senior airport security force official Mohammad Irfan told AFP.

British Airways cancelled its six weekly flights to Islamabad earlier this week, citing security reasons.

In Islamabad, officials said security had been raised significantly at the entrance and exit gates after a caller made a specific warning about an attack on the airport on Thursday.

"The airport received a telephone threat that a bomb could go off inside the building about midday," Islamabad airport official Mohammad Malik told AFP.

"Passengers were evacuated for a short time for debriefing on measures that security forces were taking because of the threat but then they were allowed to return. But there was no delay to flights," Malik said.

In February 2007 a suicide attacker opened fire near the VIP area of the same airport before blowing himself up with a hand grenade, injuring three people.

The airport, which was recently renamed after slain former premier Bhutto, lies on the outskirts of the garrison city of Rawalpindi, about 12 kilometres (eight miles) from Islamabad, and is outside the capital's main security cordon.

Islamabad itself has been under very heavy security since the Marriott blast, which turned the landmark building into a charred shell and sparked fears of further attacks.

The US embassy in Islamabad announced on Thursday it was halting its consular services amid the turmoil.

"The embassy is temporarily suspending routine consular services including visa services beginning Thursday. Emergency American citizens' services will continue to be available," spokesman Lou Fintor told AFP.

He said the embassy had also banned staff from staying at major hotels in the eastern city of Lahore, following similar edicts for Islamabad, the southern port city of Karachi and the northwestern city of Peshawar.

Meanwhile 16 Al-Qaeda-linked militants and two civilians were killed in continuing clashes between Islamist extremists and security forces in the troubled tribal belt bordering Afghanistan.

The casualties happened in the Bajaur tribal region, where the army launched an operation in August to deflect US pressure to tackle the militants. More than 800 people, mostly civilians, have been killed in the offensive.

Pakistan's tribal regions have been wracked by violence since thousands of Taliban and Al-Qaeda rebels sneaked into the country after the US-led invasion of Afghanistan in late 2001.

Pakistan's military said it had fired warning shots at two helicopters from the NATO-led force in Afghanistan after they violated Pakistani airspace on Thursday.

NATO confirmed that shots were fired but said the helicopters were inside Afghan territory. There were no casualties or damage, the NATO-led International Security Assistance Force said.