The US State Department says it has "credible" information that terrorists are planning imminent attacks against American targets around the world.

The warning comes amid fears that terror groups might try to stage attacks to coincide with the Independence Day holiday on 4 July.

Such actions may be imminent and include suicide operations

State Department warning

The State Department said attacks could include suicide missions and kidnappings, and warned US citizens and officials to be on their guard.

It said terrorists could turn to soft targets - such as churches, restaurants and clubs - as security is intensified at higher-profile places.

But despite the increased security, it has emerged that federal undercover operators managed to smuggle fake bombs and other weapons past checkpoints at 32 major US airports in June.

'Avoid crowds'

The State Department said it did not have specific information about planned attacks, but added that American citizens should remain vigilant.

Authorities fear terrorists might try to attack landmarks

It urged US nationals to be aware of their security in places where there were large numbers of Americans, and to look for alternative venues.

It said the warning superseded an earlier alert issued in March.

The previous caution was released after the wife and stepdaughter of a US diplomat were killed in a grenade attack on a church in the Pakistani capital, Islamabad.

As preparations step up for Independence Day celebrations, officials said the FBI would guard 4 July festivities and parades.