The guerrillas refused to hand over the remaining captives, two Americans and a Briton, until two ethnic Ijaw leaders were freed from jail and the global energy giant Shell pays compensation to polluted Nigerian villages.
The released hostages included American Macon Hawkins, Bardese Mohammed and Aly Shady from Egypt, Tony Santos of the Philippines and Thailand's Muado Somsak and Arak Suwana.
The freed captives, who spent 13 days in the Niger Delta swamps, were welcomed by State Governor James Ibori to his lodge in the city of Warri.
"We are very pleased that six of the hostages have been released. I want to thank everyone that has assisted in this effort. I want to thank the captors for deeming it fit to release six out of the nine," Mr Ibori said.
“But I want to appeal to them to release the others. I want to tell them that there is no political gain for holding on to the remaining three for more days any longer," he said. The former hostages did not speak to the media.
Rebel demands
One of the group, Mr Hawkins, who suffers from diabetes and high blood pressure, was released on his 69th birthday.
An emailed statement from the rebel Movement for the Emancipation of the Niger Delta (MEND) said Mr Hawkin's release was prompted by his state of health.
"He was released on account of his age and poor health with a stern warning not to return to the Niger Delta unless as a visitor," it said.
"He was released to a group of foreign journalists found to be touring the Niger Delta. No ransom for him or any other hostage has been demanded or received," the statement continued.
Of the nine oilmen seized on February 18, Cody Oswald and Russel Spell from the US and British security expert John Hudspith still be held hostage.
The group was taken when armed guerrillas stormed a pipe-laying barge operated by US engineering firm Willbros, during a series of attacks around Shell's Forcados oil terminal.
While MEND did not demand a ransom for the captives, it is insisting that the three men still being held won’t be freed until oil giant Shell pays US$1.5 billion (A$2.01 billion) in compensation to polluted Ijaw communities.
The militant group also demanded that the Nigerian government release two prominent Niger Delta leaders from jail.
Liberation fighters
Nigeria's federal government has condemned the rebels are criminals. But the Ijaw chief chosen to head the committee to negotiate the hostage’s release praised the separatists and their fight for a greater share of the nation’s oil revenues.
"The point has to be made that the young men who were involved in the kidnapping are responsible people fighting for the liberation of their people," Chief Edwin Clark told reporters at the governor's lodge.
Nigeria is Africa's biggest oil exporter, producing 2.6 million barrels per day. But because of fresh attacks on its plants, Shell has been forced to cut its output by 455,000 barrels per day.