The US-led fight against the Islamic State group in Iraq and Syria could be completed within 14 months, a senior US official told skeptical lawmakers Tuesday.

Some 65 countries are engaged at varying levels in trying to defeat the IS group in its so-called caliphate in Iraq and Syria, under a campaign that began in August 2014 and was initially devised to take three years.

US lawmakers, especially Republicans, have frequently blasted President Barack Obama for what they call an overly cautious approach that only takes incremental steps in ramping up the fight.

"We're not going to defeat them within 14 months, are we?" Senator Ron Johnson asked Brett McGurk, who is Obama's special envoy for the coalition.

McGurk, speaking at a Senate Foreign Relations Committee hearing, pointed to progress in the 22-month-old operation and said he wanted it finished even before it hits the three-year mark.

"I want it to go a lot faster than that," he told senators.

"We are moving at a tempo that I believe will lead to the ultimate defeat" of IS, he added.

Within the past week, Iraqi security forces have recaptured Fallujah from the violent extremists, and are looking to eventually seize Mosul.

In northern Syria, US-backed local fighters are focusing on the city of Manbij before an eventual battle for Raqa, the IS group's de facto capital.

The Obama administration is struggling with how best to characterize the fight.

On the one hand, the IS group has lost large portions of land and thousands of its fighters have been killed.

But CIA Director John Brennan last week warned the jihadists' "terrorism capability and global reach" had not been reduced and the number of terror attacks claimed or inspired by the group overseas keeps rising.

His comments came days after a gunman who pledged allegiance to the IS group slaughtered 49 people in a Florida gay nightclub.

Still, McGurk told lawmakers that external financing for IS had been severed, and said the jihadists' online propaganda was now being successfully countered by a global network of people, groups and companies.

He also said IS leaders were being killed at a rate of one every three days.

"And ISIL's territory is shrinking, losing nearly 50 percent of territory it once controlled in Iraq and 20 percent in Syria over the last 18 months," McGurk added.

Much of the counter-IS fight has been conducted through air strikes, though coalition forces are also training and equipping local troops in both Iraq and Syria.

Pentagon officials this week said that in northern Syria, a program to instruct leaders of anti-IS units had trained less than 100 men -- but each has a "force multiplier" effect reaching about 10,000 anti-IS fighters.

"We have a system now that is very well-structured in terms of a force that we work with on the ground, being able to call in precision air strikes," McGurk said.