A new U.S. Senate report says troops were within reach of capturing Osama bin Laden in Afghanistan's Tora Bora mountains, but the American military did not deploy thousands of troops to find him, instead choosing to keep specialized soldiers on the sidelines.

The 50-page report, called "How we failed to get bin Laden," says the failure to capture him when he was most vulnerable in December 2001 laid the foundation for today's Afghan insurgents and aggravated the internal strife afflicting Pakistan.

It says that in December 2001, bin Laden and bodyguards "walked unmolested out of Tora Bora and disappeared into Pakistan's unregulated tribal area."

But instead of deploying an attack with thousands of soldiers, which was within the means of the American military, the report found fewer than 100 U.S. commandos were assigned to work with Afghan militias to find him.

"The vast array of American military power, from sniper teams to the most mobile divisions of the Marine Corps and the Army, was kept on the sidelines," the report said.

At the time, Bush's defence secretary, Donald Rumsfeld, expressed concern that a large U.S. troop presence might fuel a backlash. He and others said the evidence was not conclusive about bin Laden's location.

But after looking at unclassified government documents and conducting interviews with officials, the report said it was "clear that Osama bin Laden was within our grasp at Tora Bora."

The report looks back at the last eight years of military operations and seeks to point the blame specifically at Rumsfeld and top military commander Tommy Franks.

It goes on to say that failing to find bin Laden "forever altered the course of conflict in Afghanistan and the future of international terrorism."

The report says that capturing the al Qaeda leader eight years ago would not have eliminated the worldwide threat of terrorism that exists today. But failing to capture him allowed bin Laden to "emerge as a potent symbolic figure who continues to attract a steady flow of money and inspire fanatics worldwide."

The report was prepared for Sen. John Kerry, chairman of the Committee on Foreign Relations, and comes out ahead of U.S. President Barack Obama's plans to send more troops to Afghanistan.

Kerry, who lost a race for U.S. President to George W. Bush in 2004, has long said Bush missed chances to get bin Laden since the September 11 attacks.

With files from The Associated Press