Militant groups in North Africa have benefited from lapses in security across the region as countries transition to more democratic government – increasingly funding themselves through kidnapping, a senior U.S. Treasury official said.

The U.S. estimates militant organizations received $120 million in ransoms over the past decade, including to al-Qaida in the Islamic Maghreb in recent years, said David Cohen, undersecretary for terrorism and financial intelligence.

Kidnapping for ransom was an "urgent threat," particularly in the Sahel, a belt of land spanning nearly a dozen of the world's poorest nations on the Sahara's southern rim, Cohen told reporters in Berlin on Tuesday.

"It is what has become perhaps the most challenging and fastest growing technique that terrorist organizations, in particular the affiliates of al-Qaida in North Africa and in Yemen, have been using to fund themselves over the last couple of years."

The Obama administration has been concerned about the growing power and influence of al-Qaida offshoots in Yemen, Somalia, Iraq and North Africa.

Small teams of special operations forces arrived at American embassies throughout North Africa in the months before militants launched the fiery attack on Sept. 11 that killed the U.S. ambassador in Libya. The soldiers' mission: Set up a network that could quickly strike a terrorist target or rescue a hostage.

The teams had yet to do much counterterrorism work in Libya, although the White House signed off a year ago on the plan to build the new military task force in the region and the advance teams had been there for six months, three U.S. counterterror officials and a former intelligence official told The Associated Press. All spoke only on condition of anonymity because they were not authorized to discuss the strategy publicly.

Officials say the military organization was too new to respond to the attack in Benghazi, where the administration now believes armed al-Qaida-linked militants surrounded the lightly guarded U.S. compound, set it on fire and killed Ambassador Chris Stevens and three other Americans.

Al-Qaida in North Africa emerged out of Algeria's civil conflict and has expanded south into the Sahara, raising its profile in recent years with hit-and-run attacks and kidnappings of westerners.

While the U.S. government has a policy of not paying ransoms, some European governments do so. The average ransom had gone up consistently over the years and was in the range of $5 million per payment, Cohen said.

Back in Washington, D.C., Republicans have questioned whether the Obama administration has been hiding key information or hasn't known what happened in the immediate aftermath of the attack.

On Tuesday, leaders of a congressional committee said requests for added security at the consulate in Benghazi were repeatedly denied, despite a string of less deadly terror attacks on the consulate in recent months. Those included an explosion that blew a hole in the security perimeter and another incident in which an explosive device was tossed over the consulate fence.

Secretary of State Hillary Rodham Clinton told Congress in a letter responding to the accusations that she has set up a group to investigate the Benghazi attack, and it is to begin work this week.

The Associated Press and Reuters contributed to this report.

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