BAGHDAD (Reuters) - A car bomb killed at least 21 people in Baghdad on Tuesday hours after Iraqi Prime Minister Ibrahim al-Jaafari angrily rejected U.S. warnings to the Shiite majority to embrace sectarian rivals in a new government.

The blast in the southern Dora district, the bloodiest in six weeks, also wounded at least 27 people, including children, a day after three bombings killed 19 people in the capital and broke a relative lull in guerrilla violence.

Iraq was counting mass casualties again as U.S. and British officials pressed its leaders to form a national unity government that can ease sectarian violence; Dora, with its mix of communities, has seen heavy bloodshed in recent months and the motivation for the latest bombing was unclear.

Speaking after talks with British Foreign Secretary Jack Straw, who echoed the U.S. ambassador's call for a government of national unity, the normally calm and diplomatic Jaafari, a Shiite Islamist, said Iraq knew its own best interests.

"When someone asks us whether we want a sectarian government the answer is 'no we do not want a sectarian government' -- not because the U.S. ambassador says so or issues a warning," he told a news conference.