"The Al-Qaeda leadership is on the run and under a lot of pressure. We are continuing to take the fight to the enemy abroad, and making it difficult for them to plan and plot against America," White House spokesman Scott McClellan told reporters, adding that the US intelligence community believes the tape is authentic.
Saudi dissident Osama bin Laden in an undisclosed place inside Afghanistan. (AFP Filed Photo)
Another top US official cautioned that the audiotape warning by Osama bin Laden aired by Al-Jazeera on Sunday should not be taken lightly.
"I believe we need to take him seriously," US ambassador to Iraq Zalmay Khalilzad said in an interview with CNN, adding that bin Laden was vying to demonstrate with the tape that "he's still a player".
"He wants to be relevant to the situation. He wants to get attention," Khalilzad said.
The Al-Qaeda chief called on Muslim fighters to go to Sudan to wage war against "crusader thieves" and slammed the international isolation of the Hamas-led Palestinian government as proof of a "war by crusaders and Zionists against Islam".
It was not clear when the tape was made, but if it proves genuine, it would be the first from bin Laden since January 19.
But the audiotape also served as a bleak reminder that, four and a half years after the September 11, 2001 attacks on the United States, the mastermind has yet to be tracked down.
Jane Harman, the top Democrat on the House of Representatives' Intelligence Committee, blamed the US military involvement in Iraq for having "bogged down" American troops and other resources and diverting attention from capturing bin Laden.
"The tape reminds us that four years after 9/11, Osama bin Laden is still at large, the subject of the largest manhunt in history, and we haven't been able to find him," said Harman on Fox News Sunday.
Republican Senator Arlen Specter also expressed frustration that so little apparent progress has been made in tracking down bin Laden.
"Frankly, I'm very dissatisfied that we haven't brought him to justice and I think it has to be a top priority," he said. Locating him, Specter added, is "a tiny needle in a giant haystack."
Democratic Senator John Kerry said that bin Laden continuing to be at-large "is one of the reasons that (US Defense Secretary) Donald Rumsfeld should resign."
Kerry, who ran against President George W. Bush in the 2004 presidential election, criticized the Bush administration for failing to capture bin Laden at Tora Bora in Afghanistan in late 2001.
Bin Laden is believed to have evaded capture by US and Afghan forces during the battle at Tora Bora and to be now hiding in Pakistan despite Bush's call for him to be hunted down "dead or alive".
Observers noted that Al-Qaeda's leader is still capable of stirring up trouble in global hotspots and remains savvy in his communications methods to get his message out.
"Al-Qaeda is very sophisticated in the communications, the words that it uses and the techniques that it uses. It's very, very good on the web, the Internet," Peter Hoekstra, the Republican chairman of the House Intelligence Committee, told Fox News Sunday.
"It recognizes that much of this war ... is about winning the hearts and the minds of moderate Islam, and they are focused on it," said Hoekstra.
The new recording called upon Muslim fighters to prepare for a prolonged war in Darfur, Bin Laden's first recent reference to Sudan, his base before he was expelled from the country in 1996 and fled to Taliban-controlled Afghanistan.
"I call upon the mujahedeen (holy warriors) and their supporters in Sudan and its surroundings -- including the Arabian Peninsula -- to prepare to lead a prolonged war against the crusader robbers in western Sudan," the voice said.
Up to 300,000 people have died and more than two million fled their homes since ethnic minority rebels in Darfur launched an uprising against the Arab-dominated regime in Khartoum more than three years ago.
But the Sudanese foreign ministry was quick to distance itself from the Bin Laden recording.
"Sudan has nothing to do with such statements," ministry spokesman Jamal Mohammed Ibrahim told AFP. Darfur is an "internal problem that we are trying to resolve under the auspices of the African Union."
The African Union is currently engaged in a diplomatic effort to bring peace to Darfur and has deployed a 7,000-strong force there. But the mission has been plagued with financial problems.
Bin Laden also savaged the West over its isolation of Hamas since the Islamic militant group took power in the Palestinian territories last month, prompting the suspension of direct aid from the United States and the European Union.
"Their rejection of Hamas after it had won the election ... confirms that there is a Crusader-Zionist war against Muslims," he said.
Hamas too sought to distance itself from the Bin Laden recording.
"What Osama bin Laden said is his opinion but Hamas has its own positions which are different to the ones expressed by Bin Laden," Hamas spokesman Sami Abu Zuhri told AFP.