"We know that [Mr Abdulmutallab] travelled to Yemen, a country grappling with crushing poverty and deadly insurgencies," said Mr Obama, who is on holiday in Hawaii.
"It appears that he joined an affiliate of al-Qaeda, and that this group, al-Qaeda in the Arabian Peninsula, trained him, equipped him with those explosives and directed him to attack that plane headed for America."
US officials have not until now publicly accused al-Qaeda over the incident on Flight 253 from Amsterdam to Detroit.
Mr Obama has come under heavy criticism from Republicans, who accuse him of not doing enough to prevent attacks on the US.
Some Democrats, too, have joined the clamour for an overhaul of intelligence procedures.
The US president - who last week ordered two security reviews - used much of his latest address to outline his administration's measures to keep America safe.
Formed in January 2009 by a merger between al-Qaeda in Saudi Arabia and Yemen
Based in eastern Yemen
Led by Nasser al-Wuhayshi, a Yemeni former aide to Osama Bin Laden. Deputy leader is Saudi ex-Guantanamo inmate Said al-Shihri
Aims to topple Saudi monarchy and Yemeni government, and establish an Islamic caliphate
Came to prominence with Riyadh bombings in 2003, and 2008 attack on US embassy in Sanaa
Blamed for attempt to blow up US passenger jet in December 2009
He said these included withdrawing troops from Iraq, boosting troop levels in Afghanistan and targeting militants in Yemen, where the suspect spent time before the attack.
"All those involved in the attempted act of terrorism on Christmas must know you too will be held to account," he said.
The US Congress, meanwhile, is preparing hearings to question intelligence officials on security failings that allowed a man whom US officials had been warned about, and who had a valid US travel visa, to allegedly smuggle explosives on to a transatlantic jet.
Mr Abdulmutallab, now in US custody, is accused of trying to detonate explosives strapped to his body as the plane, with nearly 300 people on board, prepared to land.
The mid-air drama on 25 December has spawned a worldwide aviation security review.
British Prime Minister Gordon Brown warned on Friday that rising Islamist militancy had turned Yemen into "an incubator for terrorism".
He announced an international conference in London in late January to discuss the danger.
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