London ? British soldiers began withdrawing Sunday from their last base in the southern Iraqi city of Basra, paving the way for fresh troop cuts and fueling worries about the security of the country’s second-largest city and the surrounding region.

U.S. and Iraqi authorities have expressed concern that a broader British drawdown could jeopardize the region’s rich oil resources and the land supply line from Kuwait to Baghdad and beyond. Some analysts also fear that British withdrawal could exacerbate a violent power struggle between rival Shiite groups in the sect’s southern heartland.

Around 550 soldiers were leaving the downtown Basra Palace, one of deposed President Saddam Hussein’s former compounds, to join 5,000 other personnel at an air base 7 miles away on the fringes of the city. Defense officials said the withdrawal was going well but could take days to complete.

The Iraqi military sent hundreds of reinforcements to the city to prevent Shiite militias and criminal gangs from expanding their influence once the British have gone.