Ship carrying 2,000 tonnes of acid overturns in Rhine river

It is not clear what caused the ship to overturn, although melting snow and rains have swollen rivers in the Western German region.

It is not yet known whether that was a factor in the accident on Germany’s longest river, northwest of Mainz.

The incident reportedly occurred around 5.00pm.

Helicopters equipped with infrared equipment took to the air to search for the missing crew, and traffic on the river, a major European transport artery, was suspended.

"Two members of the crew were rescued but two others are missing," police spokesman Paul-Heinz Meurisch said.

The ship measures roughly 100 metres (yards) long and was completely turned over, with its keel above the waterline, but it was not immediately known if any acid had leaked into the river, Meurisch said.

The accident occurred near a celebrated rock outcropping known as "Lorelei," a nymph from German mythology, which was said to attract mariners with enchanting songs.

The site sits above a particularly narrow point in the river where the current is very strong and many accidents have taken place in the past.