US military commanders say the al-Qaeda group has regained power in northern Iraq after being forced out of the western province of Anbar and from around Baghdad last year.
The US military does not have a large presence in Ninawa, of which Mosul is the capital.
It was unclear what role US forces would play in the offensive, which appears to be an Iraqi-led operation.
Deadly bombing
Al-Maliki's announcement of a new offensive against al-Qaeda came after a blast killed 40 people and wounded 220 in Mosul on Wednesday.
The explosion was in an unoccupied building that officials said was being used by al-Qaeda to store weapons and explosives.
Mohammed al-Askari, defence ministry spokesman, said tanks, armoured vehicles and helicopters were being sent to Iraq's third largest city, 390km north of Baghdad, for an offensive that he said would begin "very soon".
"We will provide everything that is needed for a major operation," he said.
Major-General Abdul-Kareem Khalaf, a spokesman for Iraq's interior ministry, said the Mosul push would include 3,000 extra police.
US and Iraqi troops have launched a series of offensives in northern provinces this year targeting al-Qaeda in Iraq.
The US military said last week that al-Qaeda fighters killed 3,870 civilians and wounded almost 18,000 last year, launching 4,500 attacks in Iraq.
Despite frequent violence in northern Iraq, overall attacks across the country have fallen by 60 per cent since last June.