al-Qaeda leader bin Laden, in an audio tape on April 15, extended a truce to Europeans if they withdrew troops from Muslim nations and said that the offer would last three months.

"To the European people ... you only have a few more days to accept bin Laden's truce or you will only have yourselves to blame," said the statement purported to be from Abu Hafs al-Masri Brigades, which claimed responsibility for the March 11 bombings in Spain.

It was not immediately possible to verify the authenticity of the statement, parts of which were published by Asharq al-Awsat. The daily said the letter was dated July 1.

"Muslims in the West should depart to Muslim states if they can," the letter said.

"Those who cannot should take precautions and live in Muslim areas, have enough food to last a month, find ways to protect themselves and their families, leave enough money in the house to last one month or longer and to pray a lot and put their fate in God's hands," it added.

Several European states had rejected the truce offer, which excluded the United States and Israel.

The CIA has said the voice on the tape was "likely" that of Saudi-born bin Laden, whose group carried out the September 2001 attacks on US cities.

AP adds: A constant stream of intelligence indicating that al-Qaeda wants to strike the United States this summer or fall has led federal officials to urge increased vigilance during the Independence Day weekend, but there is no specific threat of an attack timed to the holiday.

In a bulletin sent to law enforcement agencies nationwide Thursday, the FBI said police should step up patrols and watch for signs of terrorist activity, including surveillance of potential targets. The FBI also listed advice on how to spot possible suicide bombers.

"We know the US homeland remains a top al-Qaeda target," the FBI said in the bulletin, sent weekly to 18,000 state and local law enforcement agencies.