Image caption Nay Win of the NLD was jailed for accusing judges of corruption

Burma has released more than 500 prisoners, including some of the country's remaining political detainees and a number of foreigners.

Officials released no list of names, but opposition groups say at least 58 political detainees have been freed.

The move comes just a week before Burma's President Thein Sein is due to visit the United Nations in New York.

Over the past year, the government has freed more than 600 political detainees as part of a series of reforms.

They included many of the best-known opponents of the government.

Our lives are destroyed, although we are still alive Nay Win, Freed member of NLD Burma's political prisoners

"We're optimistic that these are the remaining political prisoners," said Naing Naing, a member of Aung San Suu Kyi's opposition National League for Democracy (NLD).

However, human rights groups have warned that the precise number of political prisoners is still not known.

"While another prisoner amnesty is welcome in principle, like everyone else we're left waiting to see the list before we assess how many political prisoners are included, what it means and how significant it is," Phil Robertson, deputy director of the Asia Division of Human Rights Watch, said in a statement.

"The problem is there is a lack of transparency from the Burma government about who is a political prisoner, where they are, and how many are left."

Estimates of the number of political prisoners remaining in Burmese jails vary, but rights groups say at least 300 were still being held before Monday's releases.

The government statement said the pardons were intended to ensure "stability of the state and eternal peace, by respecting humanitarian grounds... and also to have friendship and goodwill in relations with neighbouring countries".

It said those released included "foreign prisoners from the prisons around the country".

NLD member Nay Win was among those who were let out of Rangoon's Insein prison.

"Our lives are destroyed, although we are still alive," he told AFP news agency.

He said he had been given a seven-year sentence in 2008 for accusing judges of corruption. Two foreigners - one Indian and one Chinese - were also freed, he said.

President Sein, who is seeking global support for his reform drive, is about to begin a visit to China before heading to the United Nations.

Meanwhile, Aung San Suu Kyi has begun a trip to the United States, her first visit to the country in two decades.

During her 18-day trip she will be presented with the Congressional Gold Medal, the highest civilian honour in the US, among other awards.