The US military in Iraq has come under Iraqi authority for the first time since the US-led invasion in 2003, as a UN mandate for foreign troops expired and bilateral military accords took effect.

Iraqi and US officials on Thursday endorsed the return of the heavily-fortified Green Zone in central Baghdad to Iraqi control under a pact between Washington and Baghdad, paving the way to restoring sovereignty.

The US force, now more than 140,000 strong, had operated in Iraq under a UN Security Council resolution which ran out at midnight on December 31, nearly six years after Saddam Hussein, the Iraqi president, was ousted from power.

"The role of the coalition forces [in the Green Zone] will be secondary, centred on training Baghdad brigade troops to use equipment to detect explosives and advising Iraqi forces," Qassim Moussawi, a spokesman of Iraqi forces in Baghdad, said.

Under the security agreement US forces across Iraq remain under US command but operations must be authorised by a joint US-Iraqi committee.

Departure time

The pact gives US troops three years to leave the country, revokes their power to detain Iraqis without an Iraqi warrant, and subjects contractors and off-duty US troops to Iraqi law.

Other US-allied troops, including 4,100 British personnel, are to leave Iraq by July.

This also means that some 15,000 prisoners held at US military detention camps must now be charged with crimes under Iraqi law or, according to the security pact, gradually released.

Al Jazeera's Omar al-Saleh says prisoners and human rights groups however have warned that the move may not ensure the detainees' safety due to many violations in Iraqi prisons.

Despite the shift in power, many Iraqis still resent what they see as a US military occupation.

Majid Mola, an engineer, said the handover of control was meaningless because people were desperate for basic services, jobs and lasting peace.

"Where are the government services? Where is the electricity? People want practical things," he said.