One of Myanmar's most prominent rebel groups on Saturday warned a ceasefire deal seen as a breakthrough in relations with the regime was "fragile", as ethnic unrest continues to cast a shadow over reforms.

The Karen National Union (KNU) signed a pact with a delegation of ministers from the new government on January 12 in a move that raised hopes of a permanent end to one of the world's longest-running civil conflicts.

"We have to make the ceasefire durable. This is a tentative step and still very fragile," said Saw David Tharckabaw, vice-president of the KNU in charge of foreign affairs.

He said charges against a senior rebel figure and continuing conflict in other ethnic areas was eroding trust in Myanmar's nominally-civilian government, which has launched a series of changes that have persuaded some Western powers to re-evaluate tough sanctions on the country.

The KNU's Mahn Nyein Maung is facing trial for treason -- for which the maximum penalty is death -- despite assurances from the government's top negotiator that he would be freed, according to Saw David Tharckabaw.

"Not much is changing so the government can't keep its promises... that is not good for us to continue trust building," he told AFP.

"Some countries say there is a great change, real change, but we have to see proof on the ground, we cannot rely on promises."

Civil war has gripped parts of Myanmar since its independence in 1948, and an end to the conflicts is a key demand of the international community.

Tentative peace deals have been inked with several rebel groups, but fighting in Kachin has caused uncertainty about the progress of the reconciliation effort.

Citing reforms, the United States announced plans to exchange ambassadors with Myanmar soon after the KNU ceasefire and a major release of political prisoners in January.

On Thursday, US assistant secretary of state for human rights Michael Posner said violence in the northern state was getting worse and called on Myanmar to address "serious human rights abuses".

Saw David Tharckabaw said the KNU would look at Mahn Nyein Maung's treatment as well as ongoing unrest in northern Kachin province, where the army is fighting another rebel group, as indications of the regime's intentions.

But he said the KNU remained committed to giving "peaceful resolution a chance" and would participate in further negotiations with the government later this month.

President Thein Sein has surprised observers by freeing hundreds of political prisoners and reaching out to the opposition. Democracy icon Aung San Suu Kyi is optimistic enough that she is seeking a seat in parliament in April 1 by-elections.