OSLO (AFP) - A Norwegian oil platform in the North Sea has been partially evacuated due to a sudden pressure change in a well, Norwegian energy group Statoil said Friday, in an incident the company, authorities and environmentalists described as "serious".

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After one of two security mechanisms aimed at protecting against explosions for unknown reasons stopped working, 89 non-essential workers on the Gullfaks C platform were evacuated Thursday afternoon, Statoil said.

"It is a serious situation," said Inger Anda, a spokeswoman for Norway's Petroleum Safety Authority.

"There is a high risk of an accident ... if they lose control of the other security barrier," she told AFP.

The incident appeared to have happened during casing, a practice involving the injection of sludge into the oil wells aimed at making them more resilient.

If this sludge leaks out, as was apparently the case at Gullfaks C, it results in a loss of control over the pressure in the well, which in a worst-case scenario can lead to an explosion.

Statoil said it was busy trying to rectify the situation, but that it would probably take "several days."

"There are no human injuries nor damage of the platform and there is no (oil) leaking from the well," company spokesman Gisle Johanson told AFP, acknowledging however that the situation was "serious."

Some 140 workers remained on the Gullfaks C platform to try to resolve the problem.

In the meantime, the platform's security is relying on a single, so-called blowout preventer at the top of the well that works by blocking unscheduled emissions of oil and gas.

Norwegian environmental protection group Bellona also described the incident as "serious."

"This is yet another example showing that security on the Norwegian continental shelf is not good enough," head of the group Frederic Hauge said in a statement.

Norway's Petroleum Safety Authority meanwhile said it had summoned Statoil executives Friday for a report on the situation.

The incident happened as oil continues to gush into the Gulf of Mexico each day from the BP-run Deepwater Horizon rig, which exploded and sank a month ago.