Explosives packed into three shopping carts ripped through a crowded market in the Iraqi capital on Thursday, killing at least 21 people and wounding 107, an interior ministry official said. And in other violence an American contractor was killed and four Iraqis died in separate attacks in the country. But the bloodiest was the market bombing which came at rush hour on the eve of the Iraqi weekend. \"Explosives loaded in three shopping carts killed 21 people and wounded 107 at the Shurt al-Raba market,\" the official said, adding that the attack happened at 6:45 pm (15:45 GMT) when the area was crowded with shoppers. \"There were a lot of people at the market because it is Thursday, the evening before the weekend\" which begins on Friday, he said. The American contractor working for USAID was killed and another US citizen wounded in a bomb attack as their armoured vehicle was leaving a Baghdad university, the US embassy and witnesses told AFP. The attack that killed the American was an improvised bomb that penetrated their armoured vehicle, a witness said. \"An American civilian working with an implementing partner of the United States Agency for International Development in Iraq was killed in a terrorist attack today in Baghdad,\" said David J. Ranz, a US embassy spokesman. \"Three additional civilians were wounded in the attack, including one American citizen,\" he said, without giving any details of the attack. A witness said that attack took place at the capital\'s Al-Mustansariyah University, just as the delegation was leaving the college in the reinforced vehicle. In other violence in central and northern Iraq on Thursday, car bombs and improvised explosives killed four people, two of them security personnel, and wounded 17 others, seven of them soldiers or policemen. The latest attacks came at a time of surging violence in Iraq, with three assaults against foreign officials in four days. On Wednesday, gunmen fired on a visiting Iranian oil delegation in Baghdad that wounded two Iraqi guards, but the foreigners were unharmed. On Monday, a roadside bomb exploded next to a French embassy car in southern Baghdad. Four French security personnel inside the armoured vehicle were unhurt, but seven Iraqis were wounded. Eight US soldiers have been killed on duty this month. Violence is dramatically down in Iraq since its peak in 2006 and 2007, but attacks against government officials and institutions, including security forces, have risen in recent months, as Iraqi leaders bicker over key security posts left vacant since a March 2010 general election. On Tuesday, two suicide car bombs ripped through a guard post killing 26 people outside the provincial governor\'s home in the city of Diwaniyah in the centre of the country. The surge in the number of attacks comes with only months to go before US forces are due to complete a pullout under the terms of a 2008 security agreement. British private security firm AKE Group said this month that attacks have been on the rise since the start of the year, with violent incidents averaging more than 10 a day in May, up from four to five a day in January.