The CIA believed the group could put between 20,000 and 31,500 fighters in the field as of September 2014
The Islamic State (IS) group's ranks have been pared back by international and local military action in Iraq and Syria to their lowest level since Washington began monitoring the group, a senior US official said on Tuesday.
The comments from Deputy Secretary of State Antony Blinken came one day before US President Barack Obama was due to convene his national security team at CIA headquarters to take stock of the anti-IS fight.
"Working by, with and through local partners, we have taken back 40 percent of the territory that Daesh controlled a year ago in Iraq and 10 percent in Syria," Blinken told US lawmakers in prepared testimony.
"In fact, we assess Daesh's numbers are the lowest they've been since we began monitoring their manpower in 2014," he added, using a different name for IS.
Blinken did not put a new figure on the size of the group's fighting force in his statement to the Senate committee overseeing funding for the state department's programme to counter violent extremism.
But in September 2014, the last estimate to which Blinken referred, a US intelligence official told AFP that the CIA believed the group could put between 20,000 and 31,500 fighters in the field, both foreign fighters and local recruits.
Since then, US-backed Iraqi and Kurdish forces have driven to retake the western Anbar province in southwestern Iraq, although a similar move to push the militants from Mosul in the north that began last year has been slow to get off the ground.
On Wednesday, Obama and his top aides are set to evaluate the progress made so far in the anti-IS fight and weigh proposals for upping the pressure on the militant group.
"The president has asked them to come to him with suggestions for how it is possible to reinforce those elements of our strategy that are showing the most success," White House spokesman Josh Earnest told reporters on Tuesday.
When asked about a possible increase in the number of US troops in Iraq, Earnest refused to say if any announcements were on the horizon, saying only that Obama would make a statement after the meeting.
"It's not uncommon for the president to make decisions in the context of these meetings," he said.
Washington has led an international coalition against the IS group in Iraq and Syria since August 2014.
The United States, which withdrew its forces from Iraq in 2011 after eight years of war, officially redeployed 3,870 troops to the insurgency-wracked country in recent months.
But the actual number is probably closer to 5,000, according to media reports.
On Monday, France said IS strongholds in Syria and Iraq "must fall" this year.
“We must make this year a major turning point in our struggle against the so-called Islamic State" and recapture both Raqqa in Syria and Mosul in Iraq, French Defence Minister Jean-Yves Le Drian told reporters during a trip to Baghdad.
The two cities fell to IS in 2014 and have since become the group’s de facto capitals in the two war-weary countries.
The announcement came as the militants suffered setbacks in Iraq but managed to recapture al-Rai, a key town near the Turkey-Syria border that fell to rival rebel groups last week. The IS resurgence appears to have prompted swift retaliation from Turkey which launched artillery strikes on IS positions, Turkish media said.