By Bill Hayton
BBC News, Hanoi
The Vietnamese authorities have told diplomats they will free a prominent democracy activist and other dissidents ahead of a visit by US President Bush.
The US Congress is due to vote on trade ties with Vietnam
Activist Pham Hong Son was jailed over four years ago after he translated and distributed an article on democracy.
Observers see the move as an attempt to placate critics before a series of crucial votes in the US Congress.
The release is part of a general amnesty of some 5,300 prisoners to mark National Day on 2 September.
Pham Hong Son was top of the lists of Vietnamese dissidents compiled by the United States and the European Union.
He had been sentenced to five years in jail after downloading an article about democracy from the US State Department website, translating it into Vietnamese and publishing it online.
According to relatives he is currently in poor health.
Another prisoner to be released is Ma Van Bay, a Protestant minister from the Hmong ethnic minority from north-western Vietnam who was sentenced to six years in jail in 2004.
He was on a list given to the Vietnamese authorities by the US ambassador-at-large for religious freedom, John Hanford, during a visit 10 days ago.
Diplomats have said they believe the releases are intended to blunt attacks on Vietnam's human rights record ahead of votes in the US Congress vital for normalising US-Vietnamese trade relations.
However, several other dissidents are not being released including a former journalist, Nguyen Vu Binh. Earlier this month five other writers were briefly detained for planning to publish an independent newspaper.
A number of other campaigners, particularly from the country's dissident Buddhist organisation, remain under house arrest.