Violence returned Thursday to the southern city of Basra, where militants 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.

The Basra battle also exposed potential security gaps around Iraq's second-largest city less than two months after a scaled-down British force handed over control to Iraqi police and military. Rival Shiite factions are locked in fierce struggles for dominance in Basra and the rest of the oil-rich south.

U.S. forces, meanwhile, have expanded offensives in central and northern Iraq, seeking to build on gains against al-Qaida in Iraq in the past year. But the latest campaigns also have driven up the military's death toll after months of decline.

A U.S. soldier was killed Thursday by a roadside bomb in Baghdad, the military reported, but gave no other immediate details. The victim's name was not given because family had not been notified.

At least 38 U.S. soldiers have been killed in January — well above the 23 in December but still sharply lower than a year ago. In January last year, 83 soldiers were killed in Iraq.

Since the beginning of the war in 2003, at least 3,942 members of the U.S. military have died. The total for January could rise; occasionally the military reports new casualties a few days after they occur.

Casualty figures were uncertain in Basra, where militants launched a 45-minute barrage at dawn with 20 Katyusha rockets hitting Britain's base at the airport. The unguided Katyusha, which can be fired from mobile launchers, is a common part of militant arsenals in Iraq and used elsewhere by militias, including Hezbollah in Lebanon.

Three British soldiers were reported wounded, but there was little damage to the base.

The British military said it responded with six high-explosive artillery shells — killing at least one person and wounding five.

An Iraqi military intelligence officer at the British base said about 10 Iraqi civilians were either killed or wounded, although he could not provide a breakdown. The officer spoke on condition of anonymity because he was not authorized to release the information.

"We regret all deaths and injuries in Basra, from whatever cause," said Capt. Finn Aldrich, a British military spokesman in Basra.

"The only people that are suffering because of these attacks are the people of Iraq. These attacks are delaying the development of Basra and the prosperity of the people of Basra. Without security and stability, there will be limited foreign investment — investment that will speed up the development of Basra," the spokesman said.

Although Iraqi forces have command of security in Basra, about 340 miles southeast of Baghdad, Britain currently maintains about 4,500 troops at the airport outside the city and can provide assistance if called. British forces have come under sporadic attacks at the base, but the numbers have fallen dramatically since the hand-over Dec. 16.

The number of British troops could be further cut to 2,000 by spring. In the months soon after Saddam Hussein was toppled, there were about 40,000 British troops in Iraq.

In Baghdad, the car bomb was parked near a bus stop used by workers in factories scattered throughout that part of the Kazimiyah district, which is home to one of the Iraq's holiest Shiite shrines.

The area, like many Shiite-dominated neighborhoods in the capital, had returned to near normal in recent months with the overall improvement of security and expansion of checkpoints and barriers. The workers were heading home under scudding clouds and traces of light snow mixed with rain — in what has been an unusually cold Baghdad winter.

The number of Iraqi civilians and security forces killed so far in January fell to at least 599, an Associated Press tally showed, the lowest monthly death toll since December 2005, and continuing a downward trend since the fall. The figure as tabulated by Iraqi officials in the ministries of Defense, Interior and Health was slightly lower, at 543.

In another part of the capital, Adnan al-Dulaimi, one of Iraq's most influential Sunni politicians, had planned to meet with Iran's ambassador to discuss the security situation and other issues. But Iraqi soldiers prevented the Shiite diplomat from reaching the talks, officials said.

Ambassador Hassan Kazemi Qomi was turned back at an Iraqi army checkpoint near al-Dulaimi's compound, according to officials from both sides.

Security has been stepped up in the volatile Adil district after car bombs and other explosives were found there in late November — prompting the arrest of al-Dulaimi's son, Maki, along with a temporary parliamentary walkout by Sunni lawmakers.

Thursday's incident, however, appeared to be a communications mix up. Al-Dulaimi's office had not given the Iraqi army proper notice about the visit, a director in the Iranian ambassador's office said, speaking on condition of anonymity in exchange for discussing diplomatic discussions.

But it was the latest point of friction between the Sunnis and the Shiite-led establishment that is tempering optimism about a broader decline in violence.

Al-Dulaimi, a staunch government critic who leads the Iraqi Accordance Front, the largest Sunni Arab bloc in parliament, complained that the government was trying to censor those who can visit him.

"I regret this matter because we want to talk with all sides in order to achieve security and national reconciliation in Iraq," he told AP Television News. "Iraq and Iran have religious, historical and trade relations. I am working to enhance these relations."