Somali pirates have hijacked a German container vessel in an attack in the Indian Ocean, a regional maritime group says.

Andrew Mwangura, of the Kenya-based East African Seafarers Assistance Programme, said pirates seized the ship on Saturday about 400 nautical miles from the southern Somali port of Kismayu, between Kenya and the Seychelles.

"We believe the German ship has 24 crew on board. We're trying to establish their identities and the name of the vessel," Mwangura told the Reuters news agency on Sunday.

Germany's foreign ministry said it was seeking "concrete evidence" that a German-flagged vessel had been seized, Reuters reported.

"The federal government is dealing with the case, and all the appropriate public authorities are participating intensively," a foreign ministry spokeswoman said.

More than 20 ships from the EU, Nato, China and Russia now patrol the waters off Somalia and around the Horn of Africa, a busy shipping lane plagued by pirates.

Shipping routes off the coasts of eastern and western Africa have become increasingly dangerous.

Dozens of ships seized by pirates have been released after owners paid tens of millions of dollars in ransom.