Bomber attacks British firm in N.Iraq, 4 killed

* British firm said to be a construction company

* Mosul, one of Iraq's most dangerous towns

MOSUL, Iraq, July 19 (Reuters) - A suicide car bomber ploughed into a convoy carrying employees of a British company in northern Iraq on Monday, killing four of them and wounding five Iraqi civilians, Iraqi security officials said.

The suicide bomber targeted the last vehicle of the convoy in restive Mosul, a dangerous city where al Qaeda remains active, and the force of the blast threw the armoured vehicle 40 metres into a ravine, killing everyone inside, police said.

"I saw the other members of the convoy bring out four dead foreign civilians from the smashed car. One of them was beheaded," an Iraqi military officer, asking not to be named, said by telephone from the site of the attack in northern Mosul.

Iraqi officials said the firm was a British construction company but its name was not immediately known and there was no independent confirmation. The British embassy had no immediate information on the incident.

The nationality of the dead foreigners was not immediately known.

Mosul is on the front line of a longstanding feud between Iraq's Arabs and minority Kurds over land, power and oil wealth.

The region is also a hub for Sunni Islamist insurgents, and the U.S. military, which will end combat operations on Aug. 31, sees Mosul as al Qaeda's last urban stronghold in Iraq.

Iraq is on high alert for insurgent attacks after a March 7 national election produced no clear winner and left the country adrift in political uncertainty.

Overall violence has dropped sharply since the height of sectarian carnage in 2006-7. But bombings and killings continue daily. (Reporting by Jamal al-Badrani; Writing by Suadad al-Salhy; editing by Michael Christie and Tim Pearce)