Update | Foreign flood filling Islamic State ranks, UN and US intelligence say

More than 1,000 foreign fighters from 80 countries are streaming into Syria and Iraq each month despite three months of persistent air strikes against Islamic State, according to US intelligence officials and the United Nations.

A UN report published in Britain's The Guardian newspaper said about 15,000 people had travelled to fight alongside Islamic State (IS) and other hardcore militant groups from "countries that have not previously faced challenges relating to al-Qaeda".

The number of foreign jihadis travelling to fight since 2010 exceeded the cumulative total of the 20 preceding years "many times", the Security Council study said. "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 said.

A US intelligence official said it had uncovered similar evidence of mass recruitment, adding: "The flow of fighters making their way to Syria remains constant, so the overall number continues to rise."

The US Central Intelligence Agency last month announced figures showing that there were about 20,000 to 31,500 IS fighters active in Iraq and Syria. A US security official estimated that close to 2,000 Westerners were among the 15,000 foreign fighters.

Previous figures showed there were 7,000 foreign jihadis fighting in March and 12,000 in July, suggesting 1,000 a month were travelling to fight.

US officials have attributed the flows to the sophisticated recruiting campaigns by groups like IS and the relative ease with which militants from the Middle East, North Africa and Europe can get to Syria.