WASHINGTON • The number of foreign fighters joining the Islamic State in Iraq and Syria (ISIS) extremist group has decreased sharply in the past year to about 200 a month, according to a US military official.

That marks a drastic decline from about a year ago when between 1,500 and 2,000 foreign fighters were joining ISIS each month, said US Air Force Major-General Peter Gersten, deputy commander for operations and intelligence for the US-led coalition, during a news briefing on Tuesday.

Earlier this month, the US State Department said the number of ISIS fighters in Iraq and Syria was lower than at any time in the past two years.

Nonetheless, Sweden said on Tuesday that it had received intelligence about a possible attack on its capital by ISIS militants, and security services said they were investigating.

The Expressen newspaper reported Swedish security police had received intelligence from Iraq that seven or eight ISIS fighters had entered Sweden with the intention of attacking civilian targets, possibly in Stockholm.

Syria has become the main global incubator for a new generation of militants as ISIS recruited as many as 31,000 foreign fighters in the past 18 months, according to a report published by a former British spy chief last year.

Maj-Gen Gersten said that the number of fighters defecting from ISIS was increasing as well, but did not give a specific number.

"We're seeing a fracture in their morale. We're seeing their inability to pay. We're seeing the inability to fight. We're watching them try to leave Daesh in every single way," he said, using an Arabic acronym for ISIS.

In Sweden, a security police spokesman said she would not comment on any specific details of a threat, but said they were working with regular police as well as national and international partners.

"Security police are working intensively to assess received information, and it is of such a nature that our judgment is that we cannot dismiss it," she said.

An Iraqi security source said six Iraqis had left Iraq last February and entered Sweden via Turkey. The ringleader is a veteran insurgent close to Abu Musab al-Zarqawi, leader of ISIS forerunner Al-Qaeda, and current ISIS leader Abu Bakr al-Baghdadi, the source said. He was imprisoned multiple times by US forces in Iraq during their occupation.

"They want to conduct special operations to force Sweden to withdraw from the international military coalition" against ISIS, the source said, referencing recent attacks in Paris and Brussels.

REUTERS