KABUL - Afghanistan has protested to neighbouring Iran over the killing of a number of its nationals by Iranian forces, the Foreign Ministry said on Wednesday.

The ministry summoned the Iranian charge d'affairs and expressed its strong concern on Tuesday about the killings which happened a day earlier on Iranian soil near the south-western Afghan province of Nimroz.

It was the second killing of Afghans by Iranian forces in less than a month, the ministry said in a statement. It did not give details of Monday's incident or numbers involved.

Afghanistan's western border is generally peaceful though smugglers occasionally clash with security forces. Iran is a conduit for drugs smuggled from Afghanistan, the world's biggest producer of opium.

Last month, an Afghan teacher was killed and two Iranian border guards were wounded in a gunbattle between Iranian and Afghan forces in Nimroz, the Afghan government said.

The clash broke out after four Iranian border guards crossed into an Afghan village and beat up a number of residents, it said.

Also in April, Afghan police said Iranian border guards killed 13 Afghan refugees on the Afghan side of the border.