An audio recording alleged to be from top al-Qaida official Ayman al-Zawahri calls on Muslims to strike the U.S. and allied nations in their own countries using suicide tactics.

This comes as the United States government raises the national terror alert amid fears that the recent wave of terrorist attacks in Saudi Arabia and Morocco could spread to the U.S.

VOA-TV?s George Dwyer has more on all this.

The authenticity of the tape has not yet been determined, but its release comes at a moment of increased concern about potential terror attacks against the United States.

On Tuesday the U.S. terror threat level was raised from yellow, or elevated status, to orange, signifying a high risk of terrorist attack. Orange is only one-step down from the highest level of alert which is red.

The decision to raise the alert level came after top government officials reviewed intelligence reports based on intercepted communications suggesting an increased possibility of terror attacks inside the United States. Undersecretary of the Department of Homeland security, Asa Hutchinson.

?This change is based upon the recent terrorist bombings in Saudi Arabia and Morocco with intelligence reports concerning anti-U.S. terrorist group intentions."

One intercept warned of "a possible devastating attack in the next 48 hours, urging all Muslims to leave all cities including Boston, New York and the commercial coastline."

As homeland security officials try to assess the credibility of the threats, major cities are stepping-up security. In Boston, extra police are out in force, and in San Francisco, some access roads to the Golden Gate Bridge have been shut down.

Code orange has been in effect in New York City since the September 11, 2001 attacks. But the new concerns will put more security personnel in Times Square and at bridges, tunnels and commuter trains.

Homeland Security officials are now encouraging state governors and local authorities all over the country to bolster security, especially in areas where large public gatherings are planned.

Al-Qaida continues to be the principal terrorist threat, but other anti-U.S. groups are of concern as well. Meantime in Saudi Arabia authorities report that three Moroccans arrested earlier this week were planning to attack landmarks in the country by using a hijacked airliner.