The Islamic State may be plotting to launch mass casualty attacks in the US, according to sources in the US intelligence community.

A senior US intelligence official told CNN that this plan would be a departure from the ISIS' current operation of engaging in lone wolf attacks.

Previously, the al Qaeda in the Arabian Peninsula (AQAP) was viewed as the lone terrorist organisation capable of launching mass casualty attacks, which the group devastatingly showed in the series of four coordinated terrorist attacks in the US on Sept. 11, 2001.

A competition for attention and recruits is happening between ISIS and AQAP, intelligence sources said, noting that last week, AQAP bomb-maker Ibrahim al-Asiri appealed online to supporters to hold lone wolf attacks.

"I think they [ISIS leaders] are taking a lot of the new recruits that don't have time to train, who have not been brought up in their systems, and they're using them to create the type of mass casualty which produces the media attention, which is exactly what they want, that shows they're still powerful," according to CNN military analyst Lt. Gen. Mark Hertling.

Meanwhile, ISIS is being reinforced by the continuing recruitment of foreign fighters. US intelligence sources said the flow of foreign fighters to Syria and Iraq is not diminishing despite the thousands of ISIS fighters believed to have been killed in coalition air strikes. There are still about 20,000 to 30,000 ISIS fighters, the sources said.

President Barack Obama unveiled last month the US four strategies against ISIS, which he calls a "terrorist organisation, pure and simple."

These include a systematic campaign of airstrikes against ISIS in Syria and Iraq. The White House said since last year the US has launched more than 5,000 airstrikes against ISIS.

The US has also increased its support to forces fighting ISIS on the ground by sending a contingent to support Iraqi forces including Kurdish fighters with training, intelligence and equipment.

Obama also said the US is doubling its efforts to cut off ISIS' funding, improve intelligence, counter ISIS' "warped ideology" and stem the flow of foreign fighters into and out of the Middle East.

The US is also providing humanitarian assistance to religious minorities that are being driven out of their countries by ISIS, Obama said.

However, the efforts by the US to train rebels in Syria to fight ISIS is in trouble as half of those who have trained are missing or have been captured during an attack by al Qaeda-affiliate Nusra Front on a rebel site, according to CNN.

A defense official said the Syrian fighters "are no longer a coherent military unit."