The army has launched an investigation into a suspected grenade attack at a military post near the northern border with Thailand.

Four soldiers were injured in explosions from what Malaysian defence officials initially said were two grenades hurled at the post on yesterday.

However, the Thai army reportedly 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," defence ministry spokeswoman Fadzlette Othman Merican told AFP.

Fadzlette dismissed reports that security had been tightened at the Ban Di Samoe outpost in Kedah, but said that officials were exercising increased vigilance.

3 injured soldiers released

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.