A video containing the new audio message appeared on the internet
A new audio message purported to be from Osama Bin Laden has urged Muslims to launch a holy war to stop the Israeli offensive on the Gaza Strip.
The message also said Israel's assault was a desperate attempt to benefit from the last days of President George Bush.
It also accused Arab leaders of colluding with the Jewish state.
The US has dismissed the tape, saying it shows the al-Qaeda leader's isolation, and looks like an attempt to raise money.
"It appears this tape demonstrates his isolation and continued attempts to remain relevant at a time when al Qaeda's ideology, mission and agenda are being questioned and challenged throughout the world," White House spokesman Gordon Johndroe said.
"This also looks to be an effort to raise money as part of their ongoing propaganda campaign," he said.
The FBI meanwhile said it saw "no specific or credible threat" in the message to the inauguration of President-elect Barack Obama.
The 22-minute audio message entitled "A call for jihad to stop the aggression on Gaza" appeared on Islamist websites on Wednesday.
If verified, it will be the first audio tape issued by the Saudi-born militant since May 2008, and comes three weeks after Israel launched its military offensive against Hamas.
Palestinian medical officials said on Wednesday that the number of Palestinians killed in the current fighting with Israel had passed 1,000. Thirteen Israelis, including 10 soldiers, have so far been killed.
"Our brothers in Palestine, you have suffered a lot.... the Muslims sympathise with you in what they see and hear. We, the mujahideen, sympathise with you also," the new message said.
"We are with you and we will not let you down. Our fate is tied to yours in fighting the Crusader-Zionist coalition, in fighting until victory or martyrdom."
There was "only one strong way to bring the return of al-Aqsa and Palestine, and that is jihad in the path of God", referring to the al-Aqsa mosque in Jerusalem, one of the holiest sites in Islam.
The speaker also warned Muslims not to rely on Arab leaders, the majority of whom he said were "allied with the Crusader-Zionist coalition".
Israel launched its campaign against militants in Gaza on 27 December
There was also an appeal for donations to finance the fight - "tithes from any of the great Muslim or Arab traders" would be enough "for jihad on all fronts".
President Bush, the message said, was leaving his successor with a "heavy inheritance" and he doubted that the US could continue fighting al-Qaeda for decades.
"If he withdraws from the war, it is a military defeat. If he continues, he drowns in economic crisis."
BBC security correspondent Frank Gardner says the voice is the same, if sounding a little tired, but the message from the world's number-one fugitive is right up to date.
Western counter-terrorism officials had been starting to wonder if he might quietly fade away, handing over to his deputy, Ayman al-Zawahiri, our correspondent says.
Probably the last thing the outgoing President Bush wanted to hear in his final week in office was the man behind the 9/11 attacks and a reminder that he has never been caught, he adds.
Bin Laden has a multi-million dollar bounty on his head. Our correspondent says that wherever he is hiding, be it in Afghanistan or Pakistan, his close followers are clearly intensely loyal.