Islamic State’s contingent of fighters in Syria and Iraq has fallen from about 31,000 to 25,000 according to a US intelligence report revealed by the White House.

Officials in Washington cited battlefield casualties and desertions to explain the roughly 20% decrease and said the report showed a US-led campaign against Isis was making progress.

US-backed security forces in Iraq, and tribal militias and moderate opposition groups in Syria, contributed, said White House spokesman Josh Earnest, alongside a US-led air campaign that has launched more than 10,000 strikes against the Islamist extremists.

The announcement came as Saudi Arabia offered to send group troops into Syria as part of “any ground operations that the coalition [against Isis] may agree to carry out”.

Earnest said the new US intelligence estimate “means they continue to be a substantial threat, but the potential numbers have declined”.

“Isis has sustained significant casualties,” Earnest said.

Ground fighting efforts by coalition partners of the United States are having an effect in the conflict against Islamic State, he said, while international efforts were beginning to stem the flow of foreigners seeking to join the movement.

“Isil 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,” said Earnest.

The new intelligence report of 19,000-25,000 Islamic State fighters in Iraq and Syria contrasts with 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 Emily Horne, a spokeswoman for the National Security Council.

Some north African jihadists who might otherwise have travelled to Syria to join Islamic State 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 Isis 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.

(The Guardian)