Top US intelligence chiefs took full blame Wednesday for failing to stop a botched Christmas Day plane bombing, admitting that they missed a slew of warning signs.

National Intelligence Director Dennis Blair and Michael Leiter, head of the National Counter-terrorism Center, made the joint admission during a grilling from the Senate's committee on homeland security and governmental affairs.

Conceding that the system "collectively failed," the two officials said the 23-year-old Nigerian charged in the attack, Umar Farouk Abdulmutallab, "should not have stepped on that plane."

The admission detailed how the United States' vast intelligence apparatus missed glaring warnings before Abdulmutallab allegedly tried and failed to set off explosives on a Northwest Airlines jet from Amsterdam to Detroit.

In their statement, Blair and Leiter said the security services had received "strategic intelligence" that the militant group Al-Qaeda in the Arabian Peninsula was planning an attack.

However, no one ruled "that the watchlisting criteria be adjusted."

"In addition, the intelligence community analysts... did not understand the fragments of intelligence on what turned out later to be Mr Abdulmutallab, so they did not push him onto the terrorist watchlist," they said.

Other red flags that were missed included testimony from Abdulmutallab's own father who was worried his son was becoming an Islamist militant.

Intelligence even included "a partial name, an indication of a Nigerian," the security chiefs said in their joint statement.

"But there was nothing that brought it all together -- nor did we do so in our analysis."

Blair and Leiter, who were joined at the hearing by homeland security chief Janet Napolitano, promised "a fresh and penetrating look at strengthening both human and technical performance."

A concrete change made in the wake of the near bombing came Tuesday when Washington officially designated Al-Qaeda in the Arabian Peninsula a terrorist organization.

The designation opens the way for the United States to try and cut financial support for the group and its leaders Nasir al-Wahishi and Said al-Shihri.

Speaking in their defense, Blair and Leiter pointed out that the National Counter-terrorism Center "receives literally thousands of pieces of intelligence information from around the world, reviews literally thousands of names, and places more than 350 people a day on the watchlist."

"Although we must and will do better, we must also recognize that not all of the pieces rise above the noise level."

Senate security committee chairman Joseph Lieberman said "clearly, there are some things about our homeland defenses that are not working as we need them to."

However, he said that domestic defenses had improved hugely since the September 11, 2001, attacks that killed nearly 3,000 people.

He pointed out that reforms in 2004 brought about integration of the country's 16 intelligence agencies, as well as creating the National Counter-terrorism Center.

"One thing we all agree about war is mistakes are constantly made and when they're made and the enemy breaks through your defenses, you're immediately tough about it," Lieberman said.

"As you said, you close the gaps, you do hold people accountable as is appropriate, and you go on with the aim of securing the country."

- AFP