A series of bombings rattled Afghanistan Saturday, increasing the death toll in the troubled country.

In eastern Afghanistan, four NATO troops were killed in what the coalition said was a roadside bombing.

Earlier, NATO said two of its members were killed in separate attacks on Friday in southern Afghanistan.

At least 220 foreign troops have been killed in Afghanistan this year.

The continuous bombings are also taking a toll on the civilian population.

Officials at Kandahar University said two students died and at least two people were injured following bombings there Saturday.

Witnesses said the first bomb was hidden on a motorcycle and that the second blast went off after a crowd started to gather around the victims.

Afghan police say one of the victims was a student who also served as the secretary to Kandahar's chief of police.

The U.S. commander of foreign troops in Afghanistan, General David Petraeus, warned last month of a tough summer ahead as insurgents bid to show they are still a potent force amid what he said was crucial progress made by the international coalition.

The Taliban announced the start of its spring and summer fighting season at the end of April.