Urging Muslims to avoid American civilian and military sites, the apparent voice of Abd Al-Aziz al-Muqrin told visitors to the Dirasat website on Tuesday that 2004 would prove the worst yet for the US.
"The Jews, the Americans and crusaders [Christians] in general will remain the targets of our coming attacks and this year, God willing, will be fiercer and harsher for them."
The al-Qaida spokesman also called on Americans to leave the Arabian peninsula and on Washington to withdraw its troops and bases from Muslim states and end its occupation of Muslim land and its support of Israel.
However, al-Muqrin denied that last week's car bombing of a Saudi security building in Riyadh was anything to do with al-Qaida.
"We are not responsible for the bombing of the emergency security building in Riyadh although we believe it is an expected result of the infidel, unjust and oppressive policy of the apostates [Saudi rulers]".
The group which claimed responsibility for last week's attack, al-Haramain Brigades, said it supported Usama bin Ladin and was acting on behalf of the international Islamist network.