Iraqi security forces carried out mass arrests in the main northern city of Mosul on Thursday as a major crackdown against Al-Qaeda entered its second day, officials said.

Prime Minister Nuri al-Maliki, who traveled to Mosul on Wednesday to spearhead "Operation Mother of Two Springs," spent the night there and vowed to rid the province of Al-Qaeda operatives, the interior ministry said.

A statement said the premier declared that the "operations will be short and specific" in targeting "terrorists and gunmen," and security forces will not allow a free reign to militants.

On Thursday, Maliki held talks with tribal leaders on the security situation in what US commanders say is Al-Qaeda's last urban bastion in the country.

About 275 people were detained overnight on top of 560 people seized since Tuesday, Defense and Interior Ministry officials said. Fifty of those arrested have since been released.

"The terrorists are hiding in residential districts of the city to avoid [confronting] security forces," Interior Ministry spokesman Major General Abdel-Kareem Khalaf said.

The US military said it was providing logistical and intelligence support for the offensive.

A curfew was in force over-night but was relaxed at daybreak and reimposed at dusk.

Defense Ministry spokes-man Major General Mohammad al-Askari said the government was keen to ensure essential supplies get in to the city.

But the security forces continued to restrict access by car, Askari said. The city has seen a string of car bombings by insurgents loyal to Al-Qaeda.

The latest crackdown was launched following the completion of a preparatory stage code-named "Lion's Roar "which was launched at the weekend, Askari said.

Residents said the city center was calm on Thursday morning with large numbers of Iraqi and US troops on the streets. Shortly after the new offensive in Mosul was announced on Wednesday, a suicide bomber detonated explosives at a wake in the overwhelmingly province of Anbar, west of the capital, killing 20 people and wounding 45.

Anbar, the largest of Iraq's 18 provinces, was once a stronghold of Al-Qaeda. But for the past two years, tribesmen and former insurgents in the province have joined US troops in the fight against the jihadists, leading to a sharp decline in violence there.

Maliki had announced plans in February for a decisive battle against Al-Qaeda and called on the population to support the security forces to get rid of "terrorists." He said he wanted to replicate in Mosul the success his aides have boasted in the main southern city of Basra where he ordered a major crackdown against Shiite militia in the last week of March.

That offensive sparked an uprising across Shiite areas of Iraq, notably the Baghdad slum district of Sadr City, where hundreds have been killed in seven weeks of deadly battles between militiamen and US troops.

A truce agreement between the Mehdi Army militia of Shiite leader Muqtada al-Sadr and the Iraqi government was reached on Saturday and was supposed to have taken full effect on Wednesday.

The spokesman of the Sadr movement in the central shrine city of Najaf, Salah al-Obeidi, said continuing US air strikes against Sadr City were impeding implementation of the truce.

"There are obstacles and violations  ... Bombardments by US forces are one of these violations," he said. "If US forces try to enter Sadr City, this will undermine the agreement."