With a potential terrorist threat looming large over Europe, the Bush administration has stepped up security.

In line with its aviation security policy, the threat level at US airports has been raised to orange (the second of five levels), which indicates a high risk of terrorist attack.

Airports will deploy additional patrol units as America celebrates its Independence Day on Wednesday, July 4.

Besides, the number of air marshals on board flights has been increased.

On July 2, a secret US report, prepared for the Department of Homeland Security (America's home ministry), warned that Al Qaeda [Images] was planning a 'terror spectacular'.

Earlier, the ABC network reported that US law enforcement officials received intelligence two weeks ago warning of terror attacks in Glasgow, Scotland, and Prague in the Czech Republic, against 'airport infrastructure and aircraft.'

Image: A lady in a salwar-kameez at the San Francisco International Airport goes through extra security checks after the US increased security at its airports following the attack on Glasgow airport.