Numerous pirate groups are holding more than 350 hostages
A Russian warship is rushing to assist an oil tanker bound for China which has been hijacked by Somali pirates.
The Marshal Shaposhnikov was heading towards the Moscow University, which was attacked 900km (560 miles) off the Somali coast, officials said.
The 23 Russian crew on board are reported to have locked themselves in the ship's radar room.
But a BBC reporter says the Russian warship is unlikely to intervene as it could put the hostages' lives at risk.
Shots were fired at the 96,000-tonne tanker from two speedboats in the dawn attack, the ship's owner said.
The BBC's East Africa correspondent Will Ross says the oil tanker is a big prize for the pirates who, based on previous hijackings, are likely to release the cargo and crew only once a multi-million-dollar ransom has been paid.
While the international war ships have prevented some attacks in the Indian Ocean and the Gulf of Aden, it is widely felt that the solution to ending piracy is on land, he says.
Over the weekend an Islamic insurgent group took control of one of the main pirate bases on the Somali coast.
The pirates had already fled and our correspondent says is not yet clear whether this was part of a wider effort by the insurgents to stamp out piracy.
For now the pirates have moved towards other bases along the coast and at sea the hijackings continue, he says.
Numerous groups of pirates are currently holding more than 350 hostages as well as about 20 ships at various bases around the country.