COPENHAGEN, Denmark -- Two freight ships collided in the Baltic Sea off southern Sweden Monday, with at least two people reported missing, Danish and Swedish authorities said. One of the vessels capsized and at least 11 vessels, one airplane and one helicopter were taking part in the search.

The Swedish Maritime Administration received a pre-dawn alarm that two cargo ships had collided south of Ystad in Sweden, close to the Danish island of Bornholm. The authority identified the ships as Karin Hoej of Denmark and the British ship Scot Carrier. The Danish ship capsized and is upside down.

Two people from the Danish ship’s crew are missing and a search is underway, it said, adding that a helicopter and nine ships from Sweden were participating in the search, along with two Danish ships. An aircraft from the Swedish Coast Guard is also on its way.

The cause of the accident is still unclear, Swedish Maritime Administration said.

The Danish vessel will be salvaged by the Swedish coast guard and taken to Sweden because oil has started to flow into the water, Sweden's TV4 said. It was not clear where the oil came from.

The Swedish Coast Guard said it was carrying out an environmental rescue operation “to prevent oil or other harmful substances from being released into the sea” but said there “currently” were no ongoing oil spills.

According to the site marinetraffic, Scot Carrier was en route from Salacgriva in Latvia to Montrose in Scotland while Karin Hoej had left Sodertalje in Sweden for Nykoebing Falster in Denmark.

There was poor visibility caused by fog in the area.