BANGKOK (REUTERS) - Myanmar's military fired warning shots at a civilian boat carrying Thai border patrol officers, security sources and a resident said on Friday (April 23), amid heightened tensions in border areas since the junta seized power nearly three months ago.

Thursday's shooting took place near the Thai village of Tha Ta Fung in Mae Hong Son province, near where thousands of ethnic Karen from Myanmar fled military air strikes last month.

Thailand prevented most from entering its territory and tens of thousands are sheltering in the jungle on Myanmar's side.

Humanitarian groups say Myanmar forces have also opened fire on boats carrying aid to the displaced in recent weeks.

A spokesman for Myanmar's junta did not answer phone calls seeking comment on the incident.

The Thai Ministry of Defence said all agencies under the ministry and the armed forces had been instructed to "be ready to handle problems and the impact from the violent situation and fighting in border areas".

The two security sources said no one was injured in the shooting at the boat, which had hoisted the Thai flag.

"The Myanmar military unit was concerned about boats sending supplies to their opponents on the other side so they signalled the boat for inspection," one of the sources said, adding that Myanmar officers had searched the vessel.

Ms Jumi, 49, a restaurant owner in the area, said the shots were fired into the water beside the boat on the Salween river.

"People are very frightened by these shootings and they don't want to take their boats out," she said.

The military has attempted to crush protests across Myanmar against its Feb 1 coup, killing hundreds and fighting with ethnic groups along the border has also escalated.

South-east Asian leaders, including Myanmar's junta chief Min Aung Hlaing, are due to meet in Indonesia on Saturday for talks on the crisis, which analysts fear could turn into an all-out civil war.

Some of Myanmar's myriad ethnic armed groups, including the Karen National Union (KNU), which controls territory on the Thai border, have vowed to back the protesters and help overturn the coup.

The KNU's head of foreign affairs Padoh Saw Taw Nee said in a message the shooting showed Myanmar's military was "very aggressive and arrogant".