All four members of the US-led military coalition died in southern Afghanistan, where much of the fighting in the more than 10-year conflict has been concentrated, the alliance said in a statement.

NATO provided no other details about the deaths, including the nationalities of the troops. The coalition normally waits for member nations to disclose those details.

So far this month, 18 NATO service members have been killed in Afghanistan.

In the northwestern province of Badghis, a roadside bomb struck a police vehicle traveling through Qadis district, killing four officers, said the provincial governor's spokesman, Sharafudin Majedi. (AP)