Also on Saturday, MEND freed six foreigners - four Italians, one American and a Croatian - whom it seized on May 1 from a facility operated by Chevron, a US oil giant.
The group's spokesman who announced the truce said that the six had been handed over to authorities of southern Bayelsa state.
Italy's foreign ministry confirmed the release and said the freed hostages were with officials of Chevron.
But in a separate incident on Saturday, armed men disguised as riot police abducted four foreign workers from the residential compound of a subsidiary of Schlumberger, the oil services giant, in the oil city of Port Harcourt, authorities said.
A security source with an oil company in the area said the four hostages were citizens of Britain, France, the Netherlands and Pakistan.
On Friday, armed men used dynamite and machineguns to seize at least three senior Indian managers of Indorama, an Indonesian chemical company, from their residence near Port Harcourt.
Diplomatic sources said the kidnappers also seized three family members, including women and children.
Yar Adua had announced a summit on the Niger Delta beginning on Monday, but the presidency postponed it on Saturday saying participants had asked for more time for consultations.
Fighters have softened their stance since Yar Adua took power on Tuesday from Olusegun Obasanjo, who called them simple criminals.
MEND said Yar Adua should not follow the example of Obasanjo, who held summits in the past but achieved little progress through them.
The group said it would talk to the government only through intermediaries it had appointed, in the presence of a neutral arbiter acceptable to all parties.