Five US soldiers have been killed in a roadside bomb attack in the northern Iraqi city of Mosul, the US military has said.
A patrol that was hit in the explosion then came under small arms fire before US and Iraqi forces secured the area, Reuters news agency reports.
Iraqi Prime Minister Nouri Maliki announced the start of a major offensive in Mosul on Friday.
The ethnically mixed city has seen a rise in violence over the last year.
US commanders have said that Mosul is now the last large city with a significant al-Qaeda presence, after militants were driven from the Iraqi capital and Anbar province.
Last Wednesday the city was rocked by an explosion at a block of flats believed to house a bomb-making factory that left at least 34 people dead and hundreds injured.
A day later, a local police chief and two officers were killed in an ambush as they toured the scene of the blast.
Mr Maliki announced then what he said would be a "decisive" operation, and Iraqi army tanks and helicopters began to arrive in the city along with troop reinforcements on Sunday.
Elsewhere in Iraq, allies of the radical Shia cleric Moqtada Sadr said on Monday that they had urged him to follow through on threats not to extend a ceasefire agreed by his militia, the Mehdi Army.
They are angry at the continued pursuit by US and Iraqi forces of so-called rogue elements not observing the ceasefire.
The cleric and his supporters have also alleged that rival Shia militias have continued to operate within the Iraqi security forces and target their followers.
The six-month truce, due to expire at the end of February, is seen as one of the key causes of a sharp drop in violence in and around Baghdad.
An increase in US troop numbers has also been credited with reducing the violence.