A PERN spokeswoman said that flows were restored at night, while the leak took place on Sunday afternoon. The operator said that during the incident it continued oil supplies to its clients - two refineries in Poland and two in Germany.

The Russian oil pipeline monopoly, Transneft, said it had resumed oil supplies via the Druzhba pipeline following "an incident" at PERN on Sunday, TASS news agency reported.

Transneft also said that supplies were suspended on Sunday at 18.50 local time (1650 GMT) and resumed on Monday at 04.05 Warsaw time, according to TASS. PERN declined to provide the exact time of the suspension.

PERN said its technicians and firemen were working on the site to guarantee its safety.

"The leak was removed and the case is being investigated," PERN said in a statement.

(Reporting by Agnieszka Barteczko in Warsaw and Vladimir Soldatkin in Moscow, editing by Louise Heavens)