BAGHDAD — An al-Qaida in Iraq front group warned today that insurgents were waiting for the right moment to retaliate against a U.S.-Iraqi security crackdown in the northern city.

Gunmen killed a policeman near his station in Mosul, when attackers opened fire with machine guns shortly before noon about 100 yards from the Hadba police directorate, officials said.

Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf ordered an immediate investigation and expressed concern that the policeman had been killed in broad daylight today in an area that was crowded with Iraqi policemen and pedestrians.

Mosul, located 225 miles northwest of Baghdad, has been dubbed by the U.S. military as al-Qaida's last major urban stronghold in Iraq. The Iraqi government launched an offensive, dubbed Operation Lion's Roar, there earlier this month to clear the area.

Officials have claimed initial success, saying more than 1,200 suspects have been detained, and Iraqi security forces have met little resistance.

But a man claiming to be a spokesman for the Islamic State of Iraq in Nineveh province, which includes Mosul, warned in a videotape posted today that insurgents were prepared to fight back.

"Until now, we have not been engaged ... because we are the ones who control the hour to start the initiative and we will choose the time for retaliation or engagement," the unidentified spokesman said, sitting with his face covered behind a table with a laptop and wearing a white Arab robe.

"We are at full strength and have not had a single soldier killed or arrested contrary to your alleged reports" of major arrests, he said.

He said that during the first week "of the so-called the Lion's Roar operation" many people from Mosul were arrested, but he claimed they were merely former army officers, university teachers and students.

The U.S. military also said an American soldier was killed and two others were wounded Sunday afternoon in a roadside bombing 110 miles south of Baghdad near the Ash Shamiyah district of the Qadisiyah province, southeast of the Shiite holy city of Najaf.