By Tim Cocks

BAGHDAD (Reuters) - Iraqi and U.S. forces have arrested about 100 suspects thought to be crucial to the operations of militant groups in al Qaeda's last haven of north Iraq, the commander of U.S. forces in the area said on Monday.

The Iraqi military launched an offensive against al Qaeda in the northern city of Mosul and surrounding Nineveh province on May 10. It has been largely Iraqi-led, with U.S. forces playing a support role.

Gunmen from Sunni Islamist al Qaeda regrouped in Nineveh after being pushed out of other parts of Iraq. The U.S. military says Mosul is its last major urban stronghold, from where its fighters still stage suicide bomb attacks and assassinations.

"They have arrested upward of about 1,250 individuals, of which about 100 are critical targets," Major-General Mark Hertling told Reuters in a telephone interview from Mosul.

"In the last several weeks, we have either captured or killed several AQI (al Qaeda in Iraq) emirs (commanders), some suicide cell leaders, some military cell leaders," he said, adding that some were from other groups with loose links to al Qaeda.

"Some of them are very senior. I'm talking about military emirs, battalion level commanders in al Qaeda," he said.