The US State Department has warned Americans around the world that there is "credible evidence" that terrorists could attack during New Year celebrations and the Muslim holy month of Ramadan.

The warning advises US citizens abroad to avoid large crowds and gatherings and to keep a low profile.

American citizens abroad are advised to contact the nearest US embassy or consulate for up-to-date information.

The State Department gave no details of its evidence, nor any indication as to which group may be at the root of the warning.

But anonymous American officials are quoted as saying that the threat is linked to Osama bin Laden, whom the US Government suspects of involvement in the bombings last year of its embassies in Kenya and Tanzania.

Similar warnings of potential terrorist threats were issued by the State Department in October and twice in November.

Such warnings have also been issued in the past in conjunction with the start of the Muslim holy month, although there has been no recent terrorist attacks by Muslims timed to coincide with Ramadan events.

The month of Ramadan began on Thursday with the sighting of the new Moon and continues until the week after the beginning of 2000.

State Department spokesman James Foley said he could not provide additional information on the latest warning.

"We believe the information we have is credible and specific," he said in a statement read by another department spokesperson.

"The location could be any area of the world and for these reasons we've decided to issue the worldwide caution," he said.

The warning comes as a casket bearing the unidentified remains of victims of the 1995 Oklahoma City bomb has been buried.

The bombing of the Alfred P Murrah Federal Building killed 168 people. Two US citizens were convicted of the crime.

About 100 people gathered on a cold morning in Oklahoma to watch the casket, which contains mainly small fragments of tissue and bone, being buried in state Capitol grounds.