WASHINGTON (REUTERS) - The Islamic State in Iraq and Syria (ISIS) has as many as 25,000 fighters in Syria and Iraq, down from a previous estimate of up to 31,000, according to a US intelligence report revealed by the White House on Thursday (Feb 4).

US officials cited factors such as battlefield casualties and desertions to explain the roughly 20 per cent decrease in fighters, and said the report showed a US-led campaign to crush the ISIS was making progress.

The new intelligence estimate "means they continue to be a substantial threat, but the potential numbers have declined,"said White House spokesman Josh Earnest. "ISIS has sustained significant casualties," he added.

Ground fighting efforts by coalition partners of the United States are having an effect in the conflict against the ISIS, he said.

US-backed Iraqi security forces and tribal militias and moderate opposition groups in Syria have contributed. So too has a US-led air campaign that has launched more than 10,000 strikes against the Islamist extremists, he said.

Finally, international efforts are beginning to stem the flow of foreigners seeking to join the movement.

ISIS is "having more difficulty than they've had before in replenishing their ranks, and we have long been aware of the need of the international community to cooperate to stop the flow of foreign fighters to the region," Mr Earnest said.

The new intelligence report of 19,000-25,000 ISIS fighters in Iraq and Syria compares to 2014 estimates of 20,000-31,000 fighters.

"The decrease reflects the combined effects of battlefield deaths, desertions, internal disciplinary actions, recruiting shortfalls, and difficulties that foreign fighters face traveling to Syria," said Ms Emily Horne, a spokesman for the National Security Council.

Some North African militants who might otherwise have travelled to Syria to join the ISIS may instead have heeded calls by the movement's leadership to head to Libya, where the Islamists are fighting to expand their grip on territory on the Mediterranean coast.

The intelligence report did not account for the ISIS's affiliates in South Asia, other parts of the Middle East and North Africa, where its Libyan branch is expanding.

There appear to be conflicting US estimates of the strength of the movement's Libyan affiliate. Defence officials put the number at some 3,000, while other US officials put it at 5,000-6,000.