The Bulgarian troops in NATO-led International Security Assistance Force (ISAF) in Afghanistan were on alert for attacks on Sunday when they killed Afghan shooter in Kandahar, the defense minister has said.

"We had information that preparations for such an invasion are underway as early as April 12. We knew how many people would be involved and so on. The targeted cities were expected to be Kandahar and Kabul," Bulgaria's Defense Minister Anyu Angelov told TV 7 morning broadcast.

He stressed that the investigation was conducted by NATO forces and the Bulgarian military acted according to the rules.

An Afghan soldier opened fire Monday upon Bulgarian NATO soldiers serving in Afghanistan, upon which he was killed.

The lone attacker shot on Bulgarian servicemen from a watchtower of in Kandahar, while they were moving from the Hero to the Lindsey base, accorging to an Afghan Army spokesperson.

The Bulgarians returned the fire, killing the attacker, with no one of them being hurt.

According to Afghanistan Army sources, the soldier was from the northern province of Takhar.

The lone attacker might have been a part of the recent Taliban upsurge in Afghanistan, according to Lieutenant-General Atanas Samandov, commander of the Bulgarian forces in Afghanistan.