The gunmen, who are still holding Vice-President Virgilio Godoy, 19 members of the National Opposition Union (UNO) political alliance and about eight journalists at a building in the capital, Managua, released the lower-level UNO politicians Humberto Dona and Roberto Moreira on health grounds.
But the leader of a group of right-wing guerrillas, who in a separate stand-off are holding hostage 18 government officials and Sandinista politicians in the remote northern village of El Zungano, backtracked on an earlier accord to free all their prisoners, adding new conditions for their release.
Jose Angel Talavera, a former Contra rebel, also known as El Chacal (The Jackal), said that their freedom still depended on the ousting of the chief of Nicaragua's armed forces, General Humberto Ortega.
His statement, made after the release of 20 of his hostages, was a reversal of a pledge made on Sunday to release all his prisoners in return for lesser government concessions.
'Their freedom depends on agreements with the government,' Mr Talavera said. Asked what such agreements would have to cover, he replied: 'The question of Humberto Ortega.'
Mr Talavera, who took his hostages last Thursday, has been demanding that Gen Ortega and the presidential chief-of-staff, Antonio Lacayo, be fired. He accuses them of conspiring to allow the former governing Sandinistas to retain power in Nicaragua despite their 1990 election defeat by President Violeta Chamorro.
The stand-off reflects the bitter divisions left by the country's brutal eight-year civil war between the Sandinistas and US-backed Contras. The war, which ended in 1990, cost some 30,000 lives. An estimated 1,400 former Contras and some demobilised Sandinistas have rearmed in recent months.
On Sunday, Mr Talavera's forces pledged to free all their hostages in return for a government promise to stop military operations in the north of the country, where Mr Talavera is based. He did not sign the accord but his brother and chief lieutenant, Salvador, did so after consulting the guerrilla leader.
In Managua, the left-wing gunmen seized Mr Godoy on Friday together with UNO leaders to press for the release of Mr Talavera's prisoners.
Earlier yesterday a representative of the Organisation of American States, Sergio Caramagna, who was part of a commission which negotiated the release of 20 of Mr Talavera's hostages on Sunday, said that he hoped the whole hostage crisis could be resolved quickly.