Washington: A day after President Barack Obama vowed to strike the Islamic State (IS) terrorists "wherever they exist" to "degrade and ultimately destroy" the militant group, a CIA assessment increased its strength over three times to 31,500.CNN cited a CIA spokesman as saying on Thursday that the Sunni terror group that calls itself the Islamic State "can muster between 20,000 and 31,500 fighters across Iraq and Syria" as against 10,000 fighters earlier estimated by US officials.These included those who were freed from prisons by Sunni militants and Sunni loyalists who have joined the fight as the group advanced across Iraq."This new total reflects an increase in members because of stronger recruitment since June following battlefield successes and the declaration of a caliphate, greater battlefield activity and additional intelligence," the CIA spokesman was quoted as saying.CNN said it was unclear how the ranks of the IS, formerly known as the Islamic State of Iraq and Greater Syria or ISIS, swelled, and whether the increased numbers included recruits from within Iraq.More than 15,000 foreign fighters, including 2,000 Westerners, have gone to Syria, CNN cited a CIA source as saying. The fighters came from more than 80 countries, the source told the channel.Obama has been reviewing options and targets with his national security team, according to White House officials."These targets have been exposed because of the president's early decision to ramp up our intelligence, surveillance and reconnaissance assets that were operating in the region," White House spokesman Josh Earnest was quoted as saying.While officials work to develop targets in Syria, the US military is flying about 60 surveillance and reconnaissance flights a day over Iraq, a US official said.Meanwhile, House Speaker John Boehner blasted Obama on Thursday for saying no US combat troops would be placed on the ground in the fight against the IS in Syria."An F-16 is not a strategy. Airstrikes alone will not accomplish what we're trying to accomplish," Boehner told reporters.Commenting on Obama's strategy to combat the IS with US airstrikes in Syria as well as more attacks in Iraq, the Council on Foreign Relations' Col. Stephen Liszewski said "US combat power can play a critical role against ISIS"."In order to be effective, this combat power will have to be applied in a patient and deliberate manner-it cannot be done in a way that is driven by the emotion of the moment," he said."Leading such a campaign will present a challenge to Mr Obama perhaps unlike that confronted by any of his predecessors," wrote Peter Baker for the New York Times."By limiting US goals in Iraq and Syria, the president makes less likely the exit he so desperately wants," wrote David Rothkopf, CEO and Editor of the Foreign Policy Group.IANS