"A helicopter made an emergency landing. Whether it is engine failure or something else, we do not know yet," Lieutenant Colonel Michael Kurilla told reporters on Saturday.
Another officer said the aircraft was an OH-58 Kiowa but could not elaborate on the circumstances that forced it to land.
Mosul has been rocked by daily attacks against US forces and is one of the main security concerns ahead of the 30 January general elections.
Also in Mosul, a US soldier was killed on Thursday and three others wounded in a bomb attack on a military convoy, the US military revealed on Saturday.
"One Task Force Olympia soldier was killed and three were wounded after their patrol was hit by a roadside bomb," a statement said.
Another US soldier was killed on the same day in Mosul in a separate incident.
Fighters are waging a campaign
to disrupt the upcoming polls
Meanwhile, a US marine assigned to the 1st Marine Expeditionary Force was killed in a so-called non-hostile accident, the US military said.
The statement gave no further details except to say the marine died while conducting security and stability operations in al-Anbar province.
Meanwhile, the Portuguese government confirmed that it would withdraw its 120-strong military police contingent from Iraq on 12 February after the holding of elections at the end of this month.
Lisbon said it would "end the mission at the planned date" for
operational planning reasons and would ensure there was "adequate coordination with the Iraqi authorities and the stabilisation force", according to a statement quoted by Lusa news agency.
Portugal had agreed to extend the tour of duty of the National Republican Guard contingent stationed in southern Iraq, which had been due to end in November last year, in order to help provide security for the polls.
In the statement, Lisbon said it would continue to support the post-war process in the country, notably by helping to train Iraqi security forces in collaboration with Nato and the newly elected Iraqi government.
Following the elections, Iraq "will enter a new phase of the
political process which will require national participation of
different nature," the text said.