BANDA ACEH, Indonesia, July 22, Reuter - A shadowy group of gunmen, saying they are fighting for independence from Indonesia, have claimed responsibility for the killing of soldiers in Indonesia's northern Aceh province.

In a letter obtained by Reuters on Sunday the National Liberation Front Aceh Sumatra said: "We want to free the Islamic state of Aceh Sumatra and separate it from the toy state of Javanese Indonesia which continually violates the rights of the Acehnese people."

Aceh, a fiercely Islamic state, has a long history of fighting for independence. It took Dutch colonial rulers decades to subjugate the region early this century.

The government has called the killings on the northern tip of Sumatra island the work of common criminals and has vowed to crush them by the end of the year.

Officials will not give a death toll but local observers say at least 30 police and soldiers and perhaps as many civilians have been shot or decapitated in recent months.

The group's letter, sent to local newspapers earlier this month and signed "Freedom for Aceh, freedom for other islands," was not published in Indonesia's strictly controlled press.

The group threatened to take action against papers which ran what it called one-sided reports from the military.

Some analysts believe the killers, officially called GPK or "security disturbing movement," might be ex-soldiers, kicked out of the army on disciplinary charges and using separatism as an excuse to take up arms against their former colleagues.

The letter was the first claim of responsibility for the wave of killings of security officials and migrants from the dominant island, Java.

Analysts say the rebels draw support from locals angry at the heavy-handed tactics of the military and play on their resentment of development programmes they believe simply enrich outsiders.

"And how are ABRI (the armed forces) trying to deal with this? By sending in more troops. The more they do that, the more support the GPK will get," said a local bus driver, a retired soldier, echoing a sentiment heard throughout Aceh.

Regional military commander Pramono said earlier this week that five battalions had been flown in to crush the rebels.