WASHINGTON: Iran's elite Quds Force and Hezbollah militants are learning from a series of botched terror attacks over the past two years and pose a growing threat to the U.S. and other Western targets as well as Israel, a prominent counterterrorism expert says.

Operating both independently and together, the militant groups are escalating their activities around the world, fueling worries in the U.S. that they increasingly have the ability and the willingness to attack the U.S., according to a report by Matthew Levitt of the Washington Institute for Near East Studies. His report points to two attacks last year - one successful and one foiled by U.S. authorities - as indications that the militants are adapting and are determined to take revenge on the West for efforts to disrupt Tehran's nuclear program and other perceived offenses.

The report's conclusions expand on comments late last year from U.S. terrorism officials who told Congress that the Quds Force and Hezbollah, which often coordinate efforts, have become "a significant source of concern" for the U.S. The Quds Force is an elite wing of Iran's powerful Revolutionary Guard, the defenders of Iran's ruling clerics and their hold on power.

The report comes amid ongoing tensions between Iran and the West, including a persistent stalemate over scheduling six-party talks on Tehran's nuclear program and anger over reports that the U.S. and Israel were behind the Stuxnet computer attack that forced the temporary shutdown of thousands of centrifuges at an Iranian nuclear facility in 2010.

More than 20 terror attacks by Hezbollah or Quds Force operatives were thwarted around the world between May 2011 and July 2012, with nine coming in the first nine months of 2012, Levitt said in the report.

"What is particularly striking is how amateurish the actions of both organizations have been: Targets were poorly chosen and assaults carried out with gross incompetence," Levitt said in the report. "But as the groups brush off the cobwebs and professionalize their operations, this sloppy tradecraft could quickly be replaced by operational success."

Levitt is a senior fellow and director of the Washington Institute's Stein Program on Counterterrorism and Intelligence. From 2005 to early 2007, he served as deputy assistant secretary for intelligence and analysis at the Treasury Department.

The two key attacks, the report said, include the plot by a Texas man to assassinate Saudi Arabia's ambassador to the United States. Manssor Arbabsiar, a U.S. citizen with an Iranian passport, pleaded guilty to conspiracy and murder-for-hire last October and told the court that Iranian military officials were involved in the planning. Iran has denied that link.

His effort was foiled when he tried to hire what he thought was a drug dealer to carry out the attack in a Washington restaurant. The man was actually a U.S. Drug Enforcement Administration confidential source.

While that plot highlighted a growing willingness to wage attacks in the U.S., a second, more successful plot in Bulgaria suggests that militants may be learning from their missteps.

Last July, a bomb killed a bus driver and five Israelis, and wounded 30 others, when it struck a tour bus in a caravan. Officials have blamed the attack on Hezbollah.

Other attacks over the past two years have also identified repeated links between Hezbollah and the Quds force - a long alliance that historically involved the Iranians arming, funding or training the Lebanon-based militants and using them as proxies.

In testimony before the Senate Homeland Security and Governmental Affairs Committee last September, Matthew Olsen, director of the National Counterterrorism Center, said "the Quds force, as well as the group that it coordinates with, Lebanese Hezbollah" posed a significant source of concern.

FBI associate deputy director Kevin Perkins added, "We look at it as a serious threat, and ... we are focusing intelligence analysts and other resources on that on a daily basis to monitor that threat."

According to Levitt, the efforts to disrupt Iran's nuclear program have only made Tehran more eager to see a successful attack carried out. He said that both Hezbollah and the Quds Force have been hampered by the increased security triggered by the 9/11 attacks.