Khalid Mohammed, Associated Press
People watch a tow truck lift a destroyed vehicle at the site of a double car bomb attack that hit a restaurant in the Shiite-majority district of Talibiya in eastern Baghdad, Iraq, Tuesday, Oct. 21, 2014.

UNITED NATIONS — A new United Nations report says the world faces a challenge of foreign fighters in terror groups on an "unprecedented scale," with about 15,000 in Syria and Iraq alone.

The report by a panel of experts monitoring al-Qaida and the Taliban, obtained Friday by The Associated Press, has been submitted to the U.N. Security Council.

"Numbers since 2010 are now many times the size of the cumulative numbers of foreign terrorist fighters between 1990 and 2010 — and are growing," the report says. The panel was set up to support the council's al-Qaida sanctions committee.

The panel says fighters from more than 80 countries working with al-Qaida associates in Syria and Iraq "form the core of a new diaspora that may seed the threat for years to come," and that domestic terrorism could rise as fighters return to their home countries.

The diversity of countries means wider sharing of skills and training on the ground in Syria and Iraq, the report warns.

"There are instances of foreign terrorist fighters from France, the Russian Federation and the United Kingdom of Great Britain and Northern Ireland operating together," it says.

The report, dated Wednesday, also says the increasingly sophisticated use of social media is giving the Islamic State group a "cosmopolitan" appearance.

But the report says it isn't clear whether the group will rise through divisions in the wider al-Qaida network to claim dominance of the movement.

The Islamic State group has alarmed the international community with its recent, sweeping gains in Syria and northwestern Iraq and with a series of videos of beheadings of foreigners.

The U.S. has been especially concerned, sponsoring a Security Council resolution in September to address the growing threat of foreign fighters in terror groups.

The resolution requires nations to bar their citizens from traveling abroad to join terrorism organizations. The U.S. and many European nations already have laws that allow them to prosecute citizens who travel to join extremist groups or even try to do so.

But the Obama administration has acknowledged that the U.N. resolution has no enforcement mechanism to make sure other countries do the same, and that the international community has no single definition of what constitutes a terrorist group.

"It is up to member states to deal with those issues at the border, and also to deal with the broader issues that come with it," a spokesman for the U.N. secretary-general, Stephane Dujarric, told reporters Friday. He did not comment further because the report was not yet public.

Associated Press writer Edith M. Lederer contributed.