The US boosted security at airports following what British police called a terrorist attack at Glasgow airport and car bombs found in London, but left the US threat level unchanged.

White House officials said they had uncovered no specific threat in the US and the colour-coded gauge of security threats was not raised.

Still, White House spokesman Tony Snow said more police and federal agents would be sent to guard airports and travelers could expect inconveniences including longer lines ahead of the busy Independence Day holiday on July 4, especially at larger airports.

'There is no indication of any specific or credible threat to the US, no change in the overall security level,' said Snow, who is with President George W Bush at his parents' summer home in Kennebunkport, Maine.

However, Snow said the Transportation Security Administration, the federal agency in charge of security at US airports, is undertaking some 'alertness-raising measures.'

'You're likely to see those (measures) in the increased presence of some TSA agents outside terminals,' Snow said. More local police will also be sent to guard airports.

Asked if the beefed-up security was prompted by the London and Glasgow incidents, Snow said: 'I think it's really a result of just trying to make sure that, when you're taking a look at developments around the world, that you're taking every necessary precaution.'

Britain raised its national security alert level to 'critical,' the highest level that indicates an attack is expected imminently.

A four-wheel-drive vehicle crashed into the main terminal at Glasgow airport and exploded in flames in an incident that Scotland's first minister said was being treated as a terrorist attack. Earlier in London, police foiled a possible Al Qaeda plot to detonate two car bombs.

Scottish police said they believed the Glasgow incident was linked to the London bomb plot. Two men were arrested after the Glasgow attack. One of them, who was badly burnt, was found to have a 'suspect device' hidden on his body. The airport was closed after the incident.

The US Department of Homeland Security said US officials were in close contact with their UK counterparts.

'Our law enforcement and intelligence officials are closely monitoring the ongoing investigations,' DHS secretary Michael Chertoff said in the statement.

'DHS and the FBI have provided updates and protective measures, guidance to our state and local homeland security and law enforcement partners.'

'We have no plans at this time to change the national threat level,' Chertoff added.

A DHS official said Chertoff had taken part in hours of talks with local, state, federal, and international officials.

'He is going back to his weekend and his holiday plans and he encourages everyone to do the same.' Reuters