BAGHDAD — British forces formally handed over responsibility Sunday for the last region in Iraq under their control, marking the start of what Britain hopes will be a transition to a mission aimed at aiding the economy and providing jobs in an oil-rich region beset by militia infighting.

With the handover of Basra, an overwhelmingly Shiite region home to most of Iraq's oil reserves, nine of the country's 18 provinces have reverted to Iraqi government control.

"I came to rid Basra of its enemies and I now formally hand Basra back to its friends," the commander of British forces in Basra, Maj. Gen. Graham Binns, said shortly before he added his signature to papers relinquishing responsibility for the region in Iraq's far south. "We will continue to help train Basra security forces. But we are guests in your country, and we will act accordingly."

Mowafaq al-Rubaie, Iraq's national security adviser, said his government was ready and called on Basra's citizens to work together.

"Your unity is essential in rebuilding your city. You have to come together and unify — Sunnis, Shiites, Muslims and non-Muslims and nationalists," he said.

Lt. Gen. Ray Odierno, the No. 2 U.S. commander in Iraq, said the handover was "the right thing to do" for southern Iraq, but American officials worry that a power vacuum could heighten the influence of Iran and threaten land routes used to bring ammunition, food and other supplies from Kuwait to U.S. troops to the north.

"What we have to watch is undue Iranian influence," Odierno told a small gathering of reporters in Baghdad.

In Iraq's far north, Turkish warplanes hit Kurdish rebel targets early Sunday, Turkey's military said, marking an escalation of force against the outlawed separatist group. An Iraqi official said the planes attacked several villages, killing one woman.

The U.S. and Iraq have urged Turkey to avoid a major operation against Kurdish bases in northern Iraq for fear of destabilizing the most stable region in the country. Turkey has massed tens of thousands of Turkish troops along the border with northern Iraq in response to a series of attacks by Kurdish insurgents in the self-ruled region.

British Foreign Secretary David Miliband, who also attended the handover ceremony, said Britain would remain a "committed friend" of Iraq.

But he acknowledged Britain was not handing over "a land of milk and honey" to local forces.

"This remains a violent society whose tensions need to addressed, but they need to be addressed by Iraqi political leaders, and it is politics that is going to have to come to the fore in the months and years ahead," he told the British Broadcasting Corp.

President Bush predicted in January that Iraq would assume control all of its provinces by November, but the target date has slipped repeatedly, highlighting the difficulties in developing Iraqi police forces and the slow pace of economic and political progress.

American forces retain control of nine of Iraq's provinces, including Baghdad and some of the country's most volatile areas, such as Diyala and Anbar.

Binns said British forces would remain to help the Iraqis.

"Our help will continue to be one of assistance, not interference. To support, not to direct. To listen, not to ignore," Binns said. "This will be achieved by actions, not just by words. This is our promise to you, the people of Basra."

In Baghdad, there was some skepticism that Iraqi forces were ready to take control in Basra, but many agreed that the handover was a positive sign.

"I hope it will be followed by similar steps across the country. Such steps are good for Iraqis," said Awatif Qazaz, a Baghdadi woman.

In a further sign of the Iraqi government gains, the first Baghdad-Basra train since the 2003 invasion left the capital Sunday for the southern city, Iraq's second largest.

Britain's participation in the U.S.-led invasion of Iraq and the ongoing presence of troops is deeply unpopular in Britain — as is the $12 billion annual cost of operations there. A total of 174 British personnel have died in Iraq since the March 2003 invasion.

British officials have said they will retain the ability to help Iraqi troops quickly if widespread violence erupts, but they are also reducing the number of troops in the country from 4,500 to 2,000 by spring. In the months soon after Saddam Hussein was toppled, there were about 40,000 British troops in Iraq.

The main players in Basra and southern Iraq are the powerful Shiite entities — the radical cleric Muqtada al-Sadr and his Mahdi Army militia; Abdul-Aziz al-Hakim, leader the largest Shiite political party and the Badr Brigade militia, which has largely been absorbed into the Iraqi security forces; and the Fadhila party, which also has its own fighters and a member as Basra's governor.

Meanwhile, U.S. forces killed six insurgents and detained 23 suspects during weekend operations in central and northern Iraq.

Near Muqdadiyah, about 60 miles north of Baghdad, U.S. troops targeted a suspected terrorist safe house. One suspect was killed as he charged troops, apparently while wearing a suicide vest. A second suspect was killed while he was putting a suicide belt on, the military said in a statement. Two other men were killed after aircraft fired on the building.

Troops also killed or detained suspects during missions in Mosul, Samarra and Mahmoudiya.