BAGHDAD — A series of car bombs in Baghdad killed at least 28 people and wounded more than 100 on Thursday, with one exploding near the "Green Zone" diplomatic complex, the latest attacks in some of the worst violence since U.S. troops left.

Iraqi police sources said one bomb exploded just 200-300 yards outside Baghdad's international zone, close to Iraq's Foreign Ministry, killing four and wounding 12 people, the sources said.

The central zone is a highly-fortified area housing Western embassies including the U.S. mission and the nearby Iraqi ministry has been a frequent target of attacks.

Militant groups, including al-Qaida, have increased attacks in recent months in an insurgency against the Shiite-led government, raising fears of a return to full-blown sectarian conflict after U.S. troops withdrew 18 months ago.

Since the start of the year, attacks using multiple car bombs have become an almost daily occurrence, killing scores of people in Iraq, including during a religious holiday last weekend when bombers targeted families celebrating outside.

The government has launched a security sweep to try to round up suspected militants and Prime Minister Nouri al-Maliki said on Wednesday the crackdown would continue.

The United States has said it will work closely with the Iraqi government to confront al-Qaida and discuss this during a visit of Foreign Minister Hoshiyar Zebari on Thursday in Washington.

Bombers targeted districts in central, eastern, northern, and southern Baghdad, including Shiite areas, police said.

An attack in Baladiyat, eastern Baghdad, killed five people and wounded 17 when a car bomb exploded near a traffic police station.

"Windows were smashed and my children started screaming and running everywhere, smoke and dust filled my house," said a man wounded by flying shards of glass. He declined to be named.

"The politicians are responsible for the deterioration in security," he said.

Another attack in al-Shurta al-Rabaa district used a bomb on the trailer of a tractor carrying gas cylinders. It killed four and wounded 18, police said.

In Husseiniya, a district on the northeastern outskirts of Baghdad, three people were killed and fourteen wounded when a mini bus driver left his vehicle armed with a bomb in a repair shop.