Four Malaysian soldiers were injured today when attackers hurled grenades at a military post near the Thai border, the defence ministry said.

"Four soldiers suffered injuries in the hand grenade attack just before dawn today," said ministry spokeswoman Fadzlette Othman Merican.

"What we know is more than one person threw two hand grenades at the post," she told AFP, adding that the soldiers had been airlifted to hospital for treatment.

"Malaysia and Thai authorities are investigating the attack. We are suprised by the incident," she said, declining to say who might be responsible.

However, the Thai army reportedly described the incident as an accident that occurred as some 50 Malaysian and Thai soldiers conducted a joint patrol.

"They are still investigating the incident but initial investigation showed it was an accident," Thai army spokesman Akara Thitrot was quoted as saying by Bernama news agency.

Fadzlette said that some 30 soldiers were stationed at the Ban Di Samoa security outpost in northern Kedah state.

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.

Relations between Malaysia and Thailand have been strained over the crisis, and tensions were heightened in 2005 when 131 Thai Muslims fled across the border saying they feared for their lives.

Malaysia, a predominantly Muslim nation, angered the Thai government by saying that they would not be forced to return, particularly if their safety could not be assured.

Residents of Thailand's southernmost provinces are mostly ethnic Malay and Muslim, and share close cultural ties with their neighbors across the border.