At least four Afghan soldiers were killed, and about a dozen people, including six civilians, wounded after a roadside bomb planted by the Taliban exploded in the Afghan capital, officials said.

The blast on Tuesday, caused by a remote-controlled bomb, targeted an Afghan army bus at about 06.45am (0215 GMT) in the western part of Kabul, the defence ministry said.

"In a remote-controlled bomb attack against an army bus at about 6:45am in Aqa Ali Shams in Kabul, four army personnel were killed and 12 wounded, including six civilians," the ministry said in a statement.

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The Taliban, who have this year stepped up their attacks against Afghan security forces, claimed responsibility via their official Twitter account.

Afghan casualties have rocketed over the past two years as NATO has handed over most combat duties to the nation's police and army.

It is the latest in a series of attacks on local security forces as foreign combat troops will withdraw from Afghanistan by the end of the year.

A residual force of 9,800 US troops will remain, however, under a US-Afghan security agreement signed last month.

The Taliban killed six Afghan police last Tuesday in the Logar province south of Kabul, a day after fighters ambushed a convoy in the north and killed 22 policemen.

The inauguration of new President Ashraf Ghani last month was marred by a spate of suicide attacks on security forces killing more than a dozen people.

The US military estimated this month that 7,000 to 9,000 Afghan police or troops had been killed or wounded so far this year.

Civilians have also been paying a heavy price in the conflict, and the number of casualties has been increasing since the drawdown of NATO troops began in earnest.

During the first six months of 2014, 1,564 civilians were killed and 3,289 injured according to the United Nations Assistance Mission in Afghanistan.

Source: Al Jazeera and agencies