Four U.S. Marines deployed in Afghanistan to train the country’s police personnel were killed on Saturday in a roadside bombing, authorities said.

The attack, the deadliest against the U.S. forces this year, came just one day after U.S. Defense Secretary Robert Gates presented the latest stats and death tolls of the wars in the Middle East. Gates underlined the fact that more American soldiers were killed in Afghanistan than in Iraq over the past month and the latest roadside bombing came to confirm his statement.

Meanwhile, U.S. and NATO troops worked together with the Afghan forces yesterday to hunt for 870 inmates who escaped from Kandahar's Sarposa Prison after a Taliban-conducted attack which set loose the detainees from the prison.

The attack was described even by NATO as a stunning success for the militants. The city of Kandahar and its surroundings is one of the key areas in the Taliban's insurgency against President Hamid Karzai and US troops

At least 1,000 prisoners were housed in the Kandahar prison when the Taliban attacked. Although some detainees chose not to flee the prison, more than 870 inmates escaped and among them were about 390 Taliban prisoners, said the police chief of Kandahar province, Sayed Agha Saqib. Last month, the detainees in Kandahar prison carried on a week-long hunger strike forcing authorities to address their demands.

The militants assaulted the prison on motorbikes and blew off the main gate using a lorry bomb allowing detainees to flee the building. About seven police and several prisoners died in the assault.