BEIJING, Sept. 29 -- U.S. President Barack Obama acknowledged Sunday that the United States had underestimated threats from the Islamic State (IS), as another terrorist group warned to launch retaliatory attacks against the West.

U.S. warplanes started pounding IS targets in Iraq and Syria respectively on Aug. 8 and Sept. 22, with help from France and Arabian countries.

Although Obama has refused to send ground forces to fight the IS, Speaker of the House of Representatives John Boehner said his country would eventually have to send in troops.

UNDERESTIMATION AND OVERESTIMATION

The United States underestimated the rise of the IS now running amok in Iraq and Syria while overestimating the Iraqi military's ability to fight the extremist group, Obama said in an interview.

"I think our head of the intelligence community, Jim Clapper, has acknowledged that they underestimated what had been taking place in Syria," he told the CBS News "60 Minutes" program.

Meanwhile, Obama said that U.S. intelligence agencies did not have "a full appreciation of the degree to which the Iraqi security forces, or the capacity to hold ground against an aggressive adversary."

Defending his military approach, the president said: "We just have to push them back, and shrink their space, and go after their command and control, and their capacity, and their weapons, and their fueling, and cut off their financing, and work to eliminate the flow of foreign fighters."

Though Congress has approved Obama's plan to train and arm vetted Syrian rebels for them to fight the IS on the ground, Boehner said the president's goal of degrading and ultimately destroying the group cannot be achieved without sending in ground troops.

"At the end of the day I think it's going to take more than airstrikes to drive them out of there," the speaker told ABC's "This Week" program on Sunday.

"We have no choice. These are barbarians. They intend to kill us, and if we don't destroy them first, we're going to pay the price," he said.

NEW THREATS TO THE WEST

The head of Syria's al-Qaida-linked Nusra Front, which was also a target of the United States and its allies, said that the airstrikes will not defeat Islamists in his country and warned that militants might launch retaliatory attacks against Western countries.

In an audio message, Abu Mohammad al-Golani urged European and U.S. citizens to denounce the U.S. actions if they wanted to keep out of the war.

"Muslims will not watch while their sons are bombed. Your leaders will not be the only ones who would pay the price of the war. You will pay the heaviest price," he said.

The Nusra Front, which was weakened this year by battles with the IS, is now coming under pressure from its own members to reconcile with the IS and join forces to fight what they describe as a "crusader" campaign against Islam.

In the message, Golani also urged Syrian rebels not to take advantage of the U.S. strikes and hit out at the IS, even if they had suffered at the hands of the hardline group.

Warplanes of the U.S.-led coalition on Sunday night intensified their attacks against the IS. The strikes hit the entrance of the Konico factory, which is the biggest gas production facility in Syria that has recently been under the terrorist group's control.

Meanwhile, according to the Britain-based Syrian Observatory for Human Rights, U.S.-led air raids hit three makeshift oil refineries in northern Syria on Sunday.Enditem