The militants said they would not hand over the remaining captives, two Americans and a Briton, until two ethnic Ijaw leaders were freed from jail and the oil giant Shell pays a hefty sum to compensate polluted villages.
James Ibori, the Delta State governor, welcomed the men, freed on Wednesday - Macon Hawkins, Egypt's Bardese Mohammed and Aly Shady, Tony Santos of the Philippines and Thailand's Muado Somsak and Arak Suwana - to his lodge in the city of Warri.
Hawkins was released on his 69th birthday, after 13 days in the Niger Delta swamps. He suffers from diabetes and high blood pressure.
"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," Ibori said. "But I want to appeal to them to release the others. I want to tell them that there is no political gain in holding on to the remaining three for more days any longer," he added.
The rebels demand compensation
to the polluted tribal communities
The hostages did not speak to the media.
Earlier, a statement from the rebel Movement for the Emancipation of the Niger Delta (MEND) had confirmed Hawkins's release and promised that five more of the oilmen would soon be on their way to Warri.
"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," a statement from the group's e-mail address said.
"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 added.
The remaining hostages are Cody Oswald and Russel Spell from the United States and British security expert John Hudspith.
Nine oilmen were seized on 18 February when armed militants stormed a pipe-laying barge operated by their employer, the US engineering firm Willbros, during a series of attacks around Shell's Forcados oil terminal.
The MEND spokesman insisted that the remaining captives would remain in the Niger Delta creeks until the group's demands are met.
"We demand the intervention of a neutral arbiter in the resolution of this conflict and reiterate our objective of totally destroying the ability of the Nigerian government to export crude oil"
The rebel statement
They have demanded the energy giant Shell pay $1.5 billion dollars in damages to polluted Ijaw communities and that government release two prominent Niger Delta leaders from jail.
"We demand the intervention of a neutral arbiter in the resolution of this conflict and reiterate our objective of totally destroying the ability of the Nigerian government to export crude oil," the statement said.
"This objective, the world has now understood, is feasible. We will commence with attacks in another area of the Niger Delta with an aim to ensuring the total discontinuation of export of onshore crude oil," it warned.
Oil production cut
Nigeria is Africa's biggest oil exporter, producing 2.6 million barrels per day, but Shell has been forced to cut its output by 455,000 barrels per day since the start of the hostage crisis because of attacks on its plants.
Africa's biggest oil exporter
produces 2.6 million barrels a day
Chevron, the US oil giant, said on Wednesday that it had shut down one of its oil production plants in the Niger Delta.
Michael Barrett, Chevron spokesman, said the firm had shut the Makaraba flow station after an unexplained leak on a crude oil pipeline connecting the plant to the Escravos export platform caused a minor spill.
The shutdown will cost Nigeria 13,000 barrels per day in lost output.
The cause of the damage has not been explained, but a Shell source told AFP that overnight the militants had continued to dynamite evacuated oil plants a short distance further south, near the rebels' suspected base.