Fuel leaked at Russia's Baltic Sea port of Primorsk, while NORSI oil refinery caught fire following a drone attack, Russian authorities said on Sunday.

Ukraine has stepped up its attacks on Russian energy infrastructure in the past month in an effort to inflict damage on Russia's key source of revenue and undercut its military might.

The governor of north-western Leningrad region Alexander Drozdenko initially said a pipeline was damaged at Primorsk, one of Russia's main oil exporting outlets.

He later said in a post on the Telegram messaging app that the pipeline was not damaged but a fuel reservoir in the port area leaked when it was hit by shrapnel.

Primorsk, one of the country's largest export gateways, which can handle 1 million barrels per day, lost at least 40% of its storage facilities in Ukrainian drone attacks last month, U.S. commercial satellite images seen by Reuters showed on Thursday.

At some point last month, around 40% of Russia's oil exporting capabilities were shut due to the attacks, the closure of the Druzhba pipeline in Ukraine and the seizure of Russia-linked tankers.

OIL REFINERY ON FIRE

Also on Sunday, the governor of Nizhny Novgorod region, Gleb Nikitin, said on Telegram that fire had broken out at Russia's NORSI oil refinery, the country's fourth-largest, after a drone attack, adding that two facilities at the plant were hit.

He said a power station and several houses were damaged during the attack although there were no injured according to preliminary information.

NORSI, which is also Russia's second-largest producer of gasoline, can process 16 million metric tons of oil per year, or around 320,000 barrels per day.

Andrey Kravchenko, the mayor of Novorossiysk city, Russia's largest port on the Black Sea, said an air alert was in effect due to the incoming drone attack threat.

Oil loadings, including at the terminal which handles Kazakhstan oil exports from the Caspian Pipeline Consortium, are usually suspended during such alerts.