A UN report says the world is facing a challenge of foreign fighters in terror groups on an "unprecedented scale", with about 15,000 in Syria and Iraq alone.

US troops clashed with suspected Sunni insurgents holed up in a mosque north of Baghdad and launched an air-to-ground Hellfire missile into the structure.

Four suicide bombers struck communities of a small Kurdish sect in north-western Iraq, killing at least 200 people and wounding 300 more, Iraqi military and local officials said.

The US military launched a major combat operation with 1,000 Marines and Iraqi soldiers today in north-western Iraq.