US and Iraqi reinforcements, along with helicopters, tanks and armoured vehicles, have converged on Mosul for what Nuri al-Maliki, the Iraqi prime minister, has pledged would be a decisive battle against al-Qaeda.
Also in Mosul on Tuesday, US soldiers and civilian workers bade farewell to five of their comrades killed the previous day.
The soldiers died when a roadside bomb blew apart the Humvee in which were riding and armed men opened fire from a mosque.
A fierce gun battle erupted as US and Iraqi soldiers secured the area, the military said.
Iraqi soldiers entered the mosque but the assailants had already fled, according to a statement.
It was the deadliest single fatal incident since six US soldiers died on January 9 in a booby-trapped house north of Baghdad.
Monday's fatalities raised the Pentagon's January death count to at least 36.
The same day, fighters also attacked four policemen heading home from work south of Mosul, killing two and wounding the other two, Ninawa provincial police said.
Meanwhile, in Baghdad, two people were killed and around 20 wounded in a series of bomb blasts on Tuesday.
In one attack, a female suicide bomber detonated an explosives belt hidden under her black robe at a checkpoint, killing at least two women and wounding five, police said.
The attack occurred just after noon as women were being searched before being allowed to enter a commercial street in the predominantly Sunni Amariya neighbourhood in southwest Baghdad, according to police officials.
Elsewhere in Iraq, police said they found the remains of 19 executed men, including 10 heads removed from their bodies.
The discovery was made on Tuesday near the town of Muqdadiya, 65km from Baghdad, Lieutenant-Colonel Ismael al-Juburi, a provincial police official, said.
He said the victims had all been killed in the past few days.
The nine victims whose bodies were found intact had been shot dead, according to Ahmed Fuad, head of the hospital morgue in Baquba.
All the victims were men, but their identities were unknown.