Simultaneous car bombs killed 13 and hurt 43 in Tunis Street in a busy commercial area of central Baghdad on Wednesday (August 16), police said. It came as U.S. and Iraqi soldiers attempted to enforced a massive security clampdown in Baghdad, which Washington says is key to stability in Iraq. Meanwhile, clashes erupted in two of Iraq's largest cities on Wednesday while bomb attacks killed 21 in the capital, highlighting the precarious security situation as U.S. and Iraqi forces try to stem sectarian violence. Fighting in Basra began with an attack on the office of the governor and governing council. Basra Governor Mohamed Alwaili said they were mainly from the powerful Bani-Asad tribe and police sources said they were avenging the killing of a leader. British troops and a column of armoured personnel carriers rushed to Basra as armed gangs fought with Iraqi forces for more than an hour in the mainly Shi'ite city, where Prime Minister Nuri al-Maliki declared a state of emergency in June. Aqil al-Furaiji, a member of the Shi'ite-led governing council, said one policeman was killed and five wounded. Television pictures showed Iraqi forces exchanging heavy fire while two British armoured personnel carriers passed by. The British military said up to 180 British soldiers and 16 Warrior armoured personnel carriers had been dispatched to back up Iraqi troops and police on the ground. Police said they killed six insurgents in the religiously divided city of Mosul, 390 km (240 miles) north of Baghdad and the scene of heavy fighting 10 days ago when U.S. and Iraqi soldiers took several hours to restore calm. U.S. officers warn Iraq could descend into civil war unless violence is curbed and last week thousands more U.S. troops were poured into Baghdad to calm its restive neighbourhoods. Washington says the capital is key to stability in the entire country. The new Shi'ite-led government has promised to reconcile Iraq's rival sects but almost three months after Maliki took power, the bloodshed continues. Earlier on Wednesday (August 16), a roadside bomb in a small flea market in east Baghdad killed eight and wounded up to 28.