WASHINGTON (AFP) – Osama bin Laden was "within the grasp" of US forces in late 2001 but escaped because then-defense secretary Donald Rumsfeld rejected calls for reinforcements, a hard-hitting US Senate report says.

The report, set for release Monday, is intended to help learn the lessons of the past as President Barack Obama prepares to announce a major escalation of the conflict, now in its ninth year, with up to 35,000 more US troops.

It points the finger directly at Rumsfeld for turning down requests for reinforcements as Bin Laden was trapped in December 2001 in caves and tunnels in a mountainous area of eastern Afghanistan known as Tora Bora.

"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 says.

"Instead, the US command chose to rely on airstrikes and untrained Afghan militias to attack Bin Laden and on Pakistan's loosely organized Frontier Corps to seal his escape routes."

Entitled "Tora Bora revisited: how we failed to get Bin Laden and why it matters today," the report -- commissioned by Senator John Kerry, chairman of the Senate Foreign Relations Committee -- says Bin Laden expected to die and had even written a will.

"But the Al-Qaeda leader would live to fight another day. Fewer than 100 American commandos were on the scene with their Afghan allies and calls for reinforcements to launch an assault were rejected.

"Requests were also turned down for US troops to block the mountain paths leading to sanctuary a few miles away in Pakistan.

"The decision not to deploy American forces to go after Bin Laden or block his escape was made by Secretary of Defense Donald Rumsfeld and his top commander, General Tommy Franks," the report says.

"On or around December 16, two days after writing his will, Bin Laden and an entourage of bodyguards walked unmolested out of Tora Bora and disappeared into Pakistan's unregulated tribal area. Most analysts say he is still there today."

Rumsfeld's argument at the time, the report says, was that deploying too many American troops could jeopardize the mission by creating an anti-US backlash among the local populace.

The report dismisses arguments at the time from Franks, Vice President Dick Cheney and others defending the decision and arguing that the intelligence was inconclusive about Bin Laden's location.

"The review of existing literature, unclassified government records and interviews with central participants underlying this report removes any lingering doubts and makes it clear that Osama bin Laden was within our grasp at Tora Bora."

The report admits that capturing or killing the Al-Qaeda leader, accused of orchestrating the September 11, 2001 attacks on the United States that killed nearly 3,000 people, would not have eliminated the worldwide extremist threat.

"But the decisions that opened the door for his escape to Pakistan allowed Bin Laden to emerge as a potent symbolic figure who continues to attract a steady flow of money and inspire fanatics worldwide," it says.

"The failure to finish the job represents a lost opportunity that forever altered the course of the conflict in Afghanistan and the future of international terrorism, leaving the American people more vulnerable to terrorism, laying the foundation for today's protracted Afghan insurgency and inflaming the internal strife now endangering Pakistan."

As Obama prepares to announce Tuesday a bold new strategy for Afghanistan, Kerry points out at the beginning of the report that when the United States went to war less than one month after the September 11 attacks, the mission was clear: to destroy Al-Qaeda and kill or capture Bin Laden.

"Today, more than eight years later, we find ourselves fighting an increasingly lethal insurgency in Afghanistan and neighboring Pakistan that is led by many of those same extremists," he says.

"Our inability to finish the job in late 2001 has contributed to a conflict today that endangers not just our troops and those of our allies, but the stability of a volatile and vital region."