YANGON, June 25 (Reuters) - Ethnic rebel guerrillas in military-ruled Myanmar have killed 27 people in two attacks on passenger buses in the past week, among the heaviest civilian casualties in decades of unrest, state media reported on Monday.

"Terrorist insurgents are trying to undermine national stability, community peace and the rule of law just for their survival and are bullying and killing innocent people," the official New Light of Myanmar said.

The paper did not identify any of the armed groups.

In the first incident, on June 21, guerrillas killed 10 people, including a Buddhist monk, when they attacked a bus in Karen state, the paper said.

The state has been riven by ethnic conflict since the former Burma's independence from Britain in 1948 and is home to guerrillas from the predominantly Christian Karen National Union (KNU).

The KNU has been fighting for an independent homeland along the Thai border for more than five decades. It is pitted against central government forces, as well as a Buddhist Karen splinter group called the Democratic Karen Buddhist Army.

In a second ambush a day later, the paper said 17 people were killed when their bus came under fire in neighbouring Kayah state, a stronghold of guerrillas loyal to the Karrenni National Progressive Party (KNPP).

Eleven people were wounded in the attacks.

Myanmar's generals, the latest in a succession of military rulers stretching back to 1962, have signed ceasefire deals with 17 major ethnic militias.

However, the KNU, KNPP and the Shan State Army in the jungle-clad mountains of the east refuse to give up their armed struggle.

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