A Russian oil tanker carrying thousands of tonnes of oil products split apart during a heavy storm on Sunday, spilling oil into the Kerch Strait, while another tanker was also in distress after sustaining damage, Russian officials said.

The vessels were in the Kerch Strait between mainland Russia and Crimea, which Russia annexed from Ukraine in 2014, when they issued distress signals.

Russian investigators opened two criminal cases to look into possible safety violations after at least one person was killed when the 136-metre Volgoneft 212 tanker, with 15 people on board, split in half with its bow sinking.

Footage on state media showed waves washing over its deck.

The Russian-flagged vessel, built in 1969, was damaged and had run aground, officials said.

Unverified video posted on Telegram showed some blackened water on stormy seas and a half-submerged tanker.

The second Russian-flagged ship, the 132-metre Volgoneft 239, was drifting after sustaining damage, the emergency ministry said. It has a crew of 14 people and was built in 1973.

Key route for exports of fuel, grain

The Kerch Strait is a key route for exports of Russian grain and is also used for exports of crude oil, fuel oil and liquefied natural gas.

Ukraine in September accused Russia at an international court of flouting sea law by trying to keep the Kerch Strait under its sole control, something Moscow dismisses at groundless.

Emergency services said one person had died, but 12 other people had been evacuated from the first tanker. Eleven of those were taken to hospital, with two in serious condition, the TASS news agency quoted Alexei Kuznetsov, an aide to the health minister, as saying.

The emergencies ministry said it was still in contact with the other tanker and its crew after the ship ran aground 80 metres from shore near the port of Taman at the south end of the Kerch Strait.

The tankers each have a loading capacity of about 4,200 tonnes of oil products.

Official statements did not provide details on the extent of the spill or why one of the tankers sustained such serious damage.

President Vladimir Putin ordered the government to set up a working group to deal with the rescue operation and mitigate the impact of the fuel spill, news agencies cited Kremlin spokesperson Dmitry Peskov as saying, after Putin met with the ministers for emergencies and environment.

Russia said more than 50 people and equipment, including Mi-8 helicopters and rescue tugboats, had been deployed to the area.

Svetlana Radionova, head of Russia's natural resources watchdog Rosprirodnadzor, said specialists were assessing the damage at the site of the incident.