Yangon (AsiaNews/Agencies) – The two main Shan rebel groups have decided to join forces to "intensify their struggle for an independent state".

The Shan State Army (SSA) and the Shan State National Army (SSNA) declared their merger last Saturday, the 47th anniversary of 'Resistance Day', at the SSA's remote headquarters said to be on the Myanmar-Thai border.

Both are splinter groups of a rebel army led by former drug lord Khun Sa and have been fighting against the military junta for about half a century.

The move comes at a time of increasing tension between Myanmar's ethnic groups and the junta, which blamed the SSA and two other rebel armies for bombings that killed at least 19 people in the capital Yangon on May 7. The groups denied the charge and instead charged the military.

"After we have merged, the chances for independence are greater," said Yod Suk, the commander of the ethnic Tai guerrilla army, after addressing 600 troops and civilians at a ceremony to honour 31 young Tai warriors who were joining the resistance movement in Shan state.

Various armed ethnic groups in the state have fought Yangon's rulers, whom they accuse of reneging on an agreement to make Shan an independent state after Britain granted independence to the former Burma in 1948.

The SSNA agreed to a ceasefire with the military government in 1995 on the condition that they did not have to lay down arms. But when the group attended a constitution-drafting National Convention, touted by Yangon as the first step in its seven-stage "roadmap to democracy", it was told by the junta to disarm. The junta stepped up the pressure by arresting 10 Shan political leaders in February this year.

""After our peaceful diplomacy failed, we've decided to work with the SSA," said SSNA leader Sai Yi.

For the SSNA, the National Convention, which has met twice in the past two years, is a "farce", and it will boycott the conference when it reconvenes later this year.

"It has all been plotted by the Burmese. There is no point participating in more meetings," said former SSNA delegate Sai Hseng Harn, who added their views were ignored at a convention where most of the 1,000-plus delegates were hand-picked by the junta.

Myanmar's opposition National League for Democracy has also refused to join the convention as long as its leader, Nobel laureate Aung San Suu Kyi, remains under house arrest at her Yangon home.

The junta insists however that the military-run process will bring democracy to a country.