Islamic State fighters in Iraq and Syria now have about 20,000 to 31,500 fighters on the ground, the US Central Intelligence Agency says, much higher than a previous estimate of 10,000.

Among those in Syria are 15,000 foreign fighters including 2,000 Westerners, a US intelligence official told the AFP news agency on Thursday.

The figures were revealed one day after President Barack Obama pledged to expand an offensive against Islamic State group fighters, a plan which foresees new air strikes against the group in Syria, expanded attacks in Iraq and new support for Iraqi government forces.

"CIA assesses the Islamic State of Iraq and the Levant can muster between 20,000 and 31,500 fighters across Iraq and Syria, based on a new review of all-source intelligence reports from May to August," Ryan Trapani, CIA spokesman, said in a statement.

"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," he said.

Senior US officials have voiced concern at the presence of foreign fighters among the group who hold Western passports, potentially enabling them to return from the battlefield prepared to carry out terror attacks in Europe or the US.

The Islamic State group has taken control of large parts of Iraq and Syria. The group's brutal campaign has included beheadings of rivals and killings of hundreds of members of Iraq's minorities.

In recent weeks, the Islamic State has released videos depicting the beheading of two American journalists in Syria.

The violent images appear to have had an impact on a formerly war-weary public, with multiple polls in recent days showing that the majority of Americans support air strikes in both Iraq and Syria.