Obama has reviewed preliminary results of probes he has ordered into the attack, and said details were becoming clear about the 23-year-old Nigerian suspect, Umar Farouk Abdulmutallab.
"We know that he traveled to Yemen, a country grappling with crushing poverty and deadly insurgencies," Obama said in his address, posted on the White House website early Saturday.
"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 previously had not said publicly that the attack was the work of Al-Qaeda, though they had noted there was a "linkage" with the terror group.
AQAP on Monday claimed the failed December 25 jet bombing in a statement picked up by US monitors.
Abdulmutallab is accused of trying to blow up the plane as it approached Detroit from Amsterdam, by setting off explosives stitched into his underwear. The attempt failed when the detonator did not set off the explosives as planned, instead igniting a fire which was put out by passengers and crew.
On Saturday, General David Petraeus, the US regional military commander, delivered a message from Obama to Yemeni President Ali Abdullah Saleh on bilateral cooperation against terror groups, official Yemeni Saba news agency said.
Security sources said Yemen had also sent army reinforcements to the eastern provinces of Abyan, Bayada and Shawba, where Al-Qaeda militants have hideouts, and raised the alert level in those regions.
Obama said that because of past attacks by the Al-Qaeda affiliate, he had, even before the Christmas Day attempt, stepped up US cooperation with insurgency-scarred Yemen.
"Training camps have been struck; leaders eliminated; plots disrupted," he said in the address.
"And all those involved in the attempted act of terrorism on Christmas must know: you too will be held to account."
With the focus of the West trained on alleged terror havens in Yemen, British Prime Minister Gordon Brown has called for an international meeting on combating extremism in the country.
Brown's office said the meeting would take place in London January 28, running "in parallel" with a conference on Afghanistan which is expected to be attended by senior ministers or leaders from over 40 nations.
Yemen has welcomed the initiative. "It's a step in the right direction that will mobilize international support for Yemeni development and its efforts to battle unemployment and the effects of poverty," the Saba news agency quoted an official Yemeni spokesman as saying.
But such moves against terrorism were overshadowed Saturday by news that Danish police shot and wounded an axe-wielding man linked to radical Somali Islamists who tried to break into the home of a cartoonist notorious for his drawing of the prophet Mohammed.
The 28-year-old Somali national was close to the Somali Shebab movement and Al-Qaeda, which was responsible for the 9/11 attacks in the United States, the internal security service PET said.
The man was charged Saturday with attempted murder of cartoonist Kurt Westergaard and a policeman.
Angered by how narrowly tragedy was averted in a country still scarred by the September 11 attacks, Obama was meanwhile spending the weekend poring over preliminary reports of two probes he demanded into the Christmas attack.
The president heads back to Washington Monday, and plans to meet heads of intelligence agencies and relevant departments Tuesday to discuss the findings.