BAGHDAD — Violence returned yesterday to the southern city of Basra, where insurgents pummeled Britain’s airport base with 20 rockets and British gunners answered with volleys of artillery. Civilians were killed and wounded in the crossfire. In Baghdad, a bomb-rigged car blew apart at a bus stop, killing at least five people in a Shiite enclave that had not seen major violence in months. The two attacks — in areas considered relatively stable — were troubling reminders that recent improvements in Iraqi security were fragile and far from deeply rooted. American forces, meanwhile, have expanded offensives in central and northern Iraq, seeking to build on gains against Al Qaeda in Iraq in the past year. But the latest campaigns also have driven up the military’s death toll after months of decline. An American soldier was killed yesterday by a roadside bomb in Baghdad, the military reported, but gave no other immediate details.