"I have trouble believing that he is able to operate sufficiently to be in a position of major command over a worldwide al-Qaida operation but I could be wrong. We just don't know," Rumsfeld said.
The US defence chief told reporters aboard his plane on his way to Pakistan that the Bush administration still considered it a priority to capture the mastermind of the 11 September 2001 attacks on the US.
Rumsfeld, who arrived on Wednesday morning for an unannounced visit to Pakistan, said he found it interesting that bin Ladin had not been heard from publicly in nearly a year.
"I don't know what it means," he said. "I suspect that in any event, if he's alive and functioning that he's probably spending a major fraction of his time trying to avoid getting caught."
Bin Ladin's whereabouts are unknown but speculation has focused on the rugged mountainous terrain along the border of Afghanistan and Pakistan.
" ... If he's alive and functioning that he's probably spending a major fraction of his time trying to avoid getting caught"
US Defence Secretary
Rumsfeld's comments echoed earlier assessments by Ryan Crocker, the US ambassador to Pakistan, but contradicted the assertion of al-Qaida's deputy leader, Ayman al-Zawahri, in a September video interview aired by Aljazeera earlier this month that bin Ladin's battle against the West was only just beginning.
The most recent al-Qaida message from bin Laden came on 27 December 2004, when Aljazeera television broadcast a videotape in which he urged the Iraqis to boycott elections the following month.
Pakistan has been a key ally in the US-led war on terror, and during his visit, Rumsfeld was to tour areas of the country that were hit by the October earthquake, which killed more than 73,000 people there.
His visit comes a day after a similar trip by Dick Cheney, the US vice president.