A call attributed to Osama Bin Laden for Islamic holy war in Sudan and other Muslim states has been disowned by the Sudanese government and Hamas.
If the tape is genuine, it is Bin Laden's first message since January
US intelligence believes the audio tape aired by an Arab TV channel is genuine, making it the fugitive al-Qaeda leader's first message since January.
The White House said it was proof he was on the run and under pressure.
US opposition politicians said it only showed up the Bush administration's failure to capture Bin Laden.
The tape was aired by Arab satellite TV channel al-Jazeera on Sunday.
In it, the speaker identified as Bin Laden described the situation in Iraq and Sudan's troubled Darfur region as evidence of a "Zionist-Crusader war against Islam", referring to Israel and Christian states.
He called for Islamist militants to prepare for a "long war against the Crusader plunderers in western Sudan".
"Our goal is not defending the Khartoum government but to defend Islam, its land and its people," he added.
Both sides in the Darfur conflict are predominantly Muslim and one rebel group is linked to a Sudanese Islamist group.
Bin Laden was based in Sudan until he was expelled following US pressure on Khartoum.
Sudan's foreign ministry was quick to distance itself from the appeal.
"Sudan has nothing to do with such statements," spokesman Jamal Mohammed Ibrahim said.
"We are not concerned with any mujahideen or any crusade or any war with the international community.
"We are keen on reaching a peaceful solution to the crisis in Darfur."
According to the speaker on the tape, the decision by major Western powers to cut funding to the Palestinian government since the militant group Hamas won elections there was further proof of an anti-Islamic campaign.
Hamas official spokesman Sami Abu Zuhri told Reuters news agency his organisation was "interested in good relations with the West".
"We call on the Western countries to reconsider their stance towards the Palestinian cause and the Muslim nation," he added.
In the US, Jane Harman, a leading Democratic member of Congress, said the tape was a sign that the largest manhunt in history had not yielded results.
"Part of the reason is because we've been bogged down in Iraq," she added.
The defeated 2004 Democratic presidential candidate, Senator John Kerry, said the failure to capture Bin Laden was one of the reasons why US Defence Secretary Donald Rumsfeld should resign.
Even some Republicans expressed their frustration, the BBC's James Coomarasamy reports from Washington.
Senator Arlen Specter said he was dissatisfied that Bin Laden had not been brought to justice but qualified this by describing him as "a tiny needle in a giant haystack".
Since leaving Sudan, Bin Laden has been staying in Afghanistan or in mountains on the Pakistani side of their shared border.