A German shipping company stopped one of its vessels on the high seas after a tip-off that it was ferrying Iranian arms to Syria, the news weekly Der Spiegel reported Saturday on its website.

The Atlantic Cruiser, owned by Emden-based Bockstiegel but leased by a Ukrainian firm called White Whale Shipping, had loaded the arms in Djibouti, where they had been brought by an Iranian vessel, the report said.

Opponents of Syrian President Bashar al-Assad's regime revealed that the Atlantic Cruiser was halted 80 kilometres (50 miles) from the Syrian port of Tartus on Friday, Der Spiegel said.

"We had been told that the cargo contained pumps and other machinery of that sort," the magazine quoted Torsten Lueddeke of freight handlers C.E.G. Bulk Chartering as saying.

Iran is under a UN arms embargo imposed in 2007 because of its nuclear activities while Western countries have declared a similar embargo against Iranian ally Syria since its bloody crackdown on dissent began a year ago.

It was not clear what had happened to the Atlantic Cruiser. At one stage it was headed for the Turkish port of Iskunderun, Der Spiegel said.