"We launched a dawn attack, took over the flow station intact and rescued some hostages. The militants sustained casualties, about 12 of them," said Brigadier-General Lawrence Ngubane, who commands the military in the region.
The casualty figure was preliminary, he added, because troops were still "mopping up" the area, located deep in the swamps of Bayelsa state in southern Nigeria.
A spokesman for Eni subsidiary Agip, which operates the oilfield station, was not immediately available for comment.
Hostage crisis
There have been conflicting reports about the number of hostages being held at the platform. A company official initially said there were 12, but Eni headquarters in Italy issued a statement saying there were 27, including 11 soldiers.
Ngubane said there were no soldiers there when his troops attacked and they found only 11 oil workers on the facility. He did not say how many were freed unharmed.
Agip was forced by the invasion to reduce oil output by 37,000 barrels a day and declare force majeure on exports from its Brass terminal, a legal step to exempt the company from its sales contracts.
The closure lifted to 711,000 barrels a day the total Nigerian oil supply disrupted by attacks and was a setback to moves by Umaru Yar'Adua, the newly-inaugurated president, to calm tensions in Nigeria's anarchic southern oil region.
Activists in the delta said the invasion was in response to the killing of eight people in a boat by troops guarding Ogbainbiri flow station on June 12.