Malaysia does not believe the Thai military is responsible for a suspected grenade attack on an army border post that injured four soldiers, a minister said today.

The army is conducting a probe into Wednesday's incident at the Ban Di Samoe outpost in northern Kedah state.

Malaysian defence officials initially said that two grenades were hurled at the post, but the Thai army said it was an accident that took place as some 50 Malaysian and Thai soldiers conducted a joint patrol.

"There are conflicting views by the two parties, so that's where we need to investigate," Malaysian defence ministry spokeswoman Fadzlette Othman Merican told AFP.

Deputy Defence Minister Datuk Zainal Abidin Zin told the state Bernama news agency that the incident was most likely the work of an individual or a group.

"Relations between Malaysia and Thailand, the relations between our soldiers and Thai soldiers, are good. Therefore, we do not harbour any preconception that the incident was the work of Thai soldiers," he said.

Zainal Abidin said a platoon of soldiers had been despatched to scour the area and trace the source of the explosions.

"As far as I know, we are getting the cooperation of the Thai government and military," he said.

The defence ministry said three of the injured soldiers had been released from a Kedah hospital and returned to the base, and that the one who remained was in a stable condition.

Malaysia tightened security on its border with Thailand last month, restricting entry and banning Thai motorcycles after a Malaysian border official was attacked by a group of Thai fuel smugglers.

Southern Thailand is in the grip of a three-year separatist insurgency, which has left 1,800 people dead.