Fuel leaked at Russia's Baltic Sea port of Primorsk, while Lukoil-Nizhegorodnefteorgsintez (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 able to handle 1 million barrels per day, lost at least 40% of its storage facilities in Ukrainian drone attacks last month, US 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 pipeline in Ukraine and the seizure of Russia-linked tankers.

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

According to Nikitin, Russian air defense forces repelled an overnight attack by 30 Ukrainian drones over an industrial zone in the Kstovsky district.

Debris from the downed drones damaged two facilities at NORSI, triggering fires that were later contained.

Nikitin said the attack also damaged a thermal power plant and several residential buildings, adding that preliminary reports indicate no casualties.

NORSI can process 16 million metric tonnes of oil per year, or around 320,000 barrels per day, according to Reuters.

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

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

(With input from Reuters, Xinhua)