US Defence Secretary Donald Rumsfeld said on Sunday that US forces are better equipped to counter al Qaeda and the Taleban in Afghanistan as US forces launched a new offensive against Osama bin Laden's fighters.
But Rumsfeld insisted that bin Laden's whereabouts remain a mystery.
"He may be alive and he may not be. We don't know if he's alive or dead," Rumsfeld told CNN television.
"He may be in Afghanistan. He may be in Pakistan. He may be someplace else."
National security advisor Condoleezza Rice also said "people should stop speculating" about media reports that US forces are closing in on the mastermind of the September 11 attacks on New York and Washington in 2001.
"I think it's really not backed up by sound intelligence," she told NBC television.
US forces have launched Operation Mountain Storm along the mountainous border with Pakistan in south and south-east Afghanistan that is believed to have been the last refuge of bin Laden.
"We are certainly better organised today, and we've put a lot of pressure on the al Qaeda network around the world. And we believe we're safer and more secure because we have put pressure on that network," said the defence secretary.
"What's going on is a normal activity that takes place," said Rumsfeld. "And from time to time, there are sweeps made. And I think to hype it or suggest that there's something major going on is probably a misunderstanding."
"I don't know if he'll be caught this year," added the defence secretary, "if he's alive, I'm sure he'll be caught eventually. And when, I don't know."
On the eve of a visit to Pakistan by US Secretary of State Colin Powell, Rumsfeld praised the "terrific co-operation" of Pakistani forces in the hunt for bin Laden.
"They're working closely with the tribal leaders in that area. We have also trained an Afghan national army, and the Afghan national army is participating with our coalition forces. And they're working their way around in that country to see that the Taleban and the al Qaeda don't have an opportunity to regroup and try to cause additional terrorist acts."
Rumsfeld said he believed Pakistan's President Pervez Musharraf was doing everything possible to aid the US war on terrorism.
"We have a lot of respect for the efforts they're making. And they've been helpful.
"They've rounded up al Qaeda from time to time and put pressure on the Taleban. And we appreciate that co-operation in the global war on terror both by the Afghan government and by the Pakistan government." Rumsfeld said Musharraf shared US concern.