(Adds detail, Ban Ki-moon comment)
BAGHDAD, Jan 13 (Reuters) - A series of car bombs killed at least 21 people in Baghdad on Monday, police said, in violence that coincided with a visit to the Iraqi capital by U.N. Secretary-General Ban Ki-moon.
The deadliest attack took place in the mainly Shi'ite district of Shaab in eastern Baghdad, where a car bomb exploded in a commercial area, killing 11 people and wounding 28.
Although no group claimed responsibility, the bombings appeared to be part of a relentless campaign by al Qaeda-linked militants to undermine Prime Minister Nuri al-Maliki's Shi'ite-led government.
Gunmen also attacked a checkpoint in southern Baghdad, killing three policemen and wounding four, police said.
Last year was Iraq's bloodiest since 2008, with nearly 9,000 people killed, according to U.N. figures.
Ban told a news conference after talks with Maliki that he was concerned about the spiralling violence and urged Iraqi leaders to deal with its underlying causes.
On Jan. 1, hardline Sunni militants overran the western cities of Falluja and Ramadi in a bold challenge to the government, alarming the United States, whose troops fought fierce battles with insurgents in Falluja in 2004.
Militants in central Falluja fired mortars at an army base outside the city on Monday and soldiers fired four shells back, killing two people and wounding eight, police and hospital sources said. There was no word on any army casualties. (Reporting by Suadad al-Salhy; Writing by Alistair Lyon; Editing by Alison Williams)