WASHINGTON, Sept. 15 — As British troops pull out of their last base in Basra, some military commanders and civilian government officials in the area are concerned that the transition could leave them and a major supply route to Baghdad at greater risk of attack.

The route, a lifeline that carries fuel, food, ammunition and equipment for the war, crosses desert territory that is home to rival militias and criminal gangs. In interviews, Americans stationed in the southern provinces and Pentagon planners say they are closely watching the situation there as the British pass security responsibility to local Iraqi units.

There is little talk of increasing the American troop presence along the major supply route, which links Baghdad and Kuwait and is called M.S.R. Tampa, although officials in Baghdad and Washington say other options include increased patrols by armed surveillance aircraft, attack helicopters and combat jets.

The significant attention being paid to security in southern Iraq came as the senior allied commander, Gen. David H. Petraeus, announced plans in Washington this week to reduce American troop presence by five combat brigades across the country by next summer.