The men blew up the Russians' homes with dynamite before taking them away, said Felix Nxong, the general manager of the plant in the southern Akwa Ibom state.
The Movement for the Emancipation of the Niger Delta (MEND) said on Saturday it would halt attacks for one month to pave the way for talks with the new Nigerian government.
MEND also 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.
However, the announcement is not likely to immediately calm the region where numerous criminal gangs and armed groups ply the swamps and creeks in gunboats.
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.
More than 200 foreigners have been kidnapped since armed men increased their attacks in late 2005 - with more than 100 foreign workers taken this year alone.
Held for ransom
Hostages are generally released unharmed after a ransom is paid.
In his inaugural speech last week, Umaru Yar Adua, Nigeria's president, called the conflict an urgent matter and asked for a permanent ceasefire as a step towards long-term peace.
Nigeria is Africa's top crude producer, an Opec member and a leading exporter of oil to the US.
The rising violence in Nigeria comes amid tight global oil supplies around the world, increased demand from China and India and turmoil in the Middle East.