Major General Carter Ham said on Wednesday that although the last four attacks on the city's stations had been repelled by Iraqi police and National Guards, he noted Mosul essentially remained without a working police force.
He added that the situation could compromise security for the 30 January elections.
Mosul has witnessed a surge in anti-US resentment after their forces launched an assault on the city of Falluja early last month. US commanders claim many fighters fled Falluja for Mosul before that offensive.
Fear and elections
An Iraqi political anaylst based in Mosul, Yunus Hashim, also told Aljazeera that an atmosphere of fear and terror pervaded Mosul due to "the deteriorating security situation and frequent killings" and that it would inevitably affect the elections.
"There are no preparations or even acceptance of the elections in the city due to the occupation," he said.
He said many candidates would have been prepared to stand for elections if they had been delayed six months, but "their hopes have faded ... many see the election of specific participants as a foregone conclusion".
Meanwhile, unknown assailants have shot and killed a US marine west of Baghdad, according to a military statement.
The marine was killed on Tuesday while carrying out operations in al-Anbar, a large province west of Baghdad that includes the volatile cities of Falluja and Ramadi.
No further details were given. About 1299 US troops have died in Iraq since its initial invasion.