Pirates are believed to use "mother-ships" to attack far from the shore
Somali pirates have attacked an oil tanker some 1,000 nautical miles (1,850km) off the coast, the EU's anti-piracy mission says.
The Hong Kong-registered BW Lion managed to evade an attack by two fast skiffs, firing automatic weapons and rocket-propelled grenades, it says.
This is said to be the furthest pirate attack yet from the Somali coast.
Meanwhile, a maritime expert reports that pirates have seized a ship carrying weapons.
Up to 40 ships from the EU, US, China, India and Japan are involved in anti-piracy operations, mainly around the Gulf of Aden.
The forces believe their presence has deterred and foiled many attacks.
But BBC defence correspondent Nick Childs says the pirates have changed tactics and shifted their focus further out into the Indian Ocean.
The BW Lion was attacked some 400 nautical miles (741km) north-east of the Seychelles.
Pirates are believed to use "mother-ships" to transport small speedboats from the coast so they can stage attacks.
Andrew Mwangura, of the East African Seafarers' Assistance Programme, told the BBC that a ship carrying weapons to Somalia in contravention of a UN arms embargo was seized on Sunday.
The ship's owners, however, insists that it was carrying food.
Last month, a British couple sailing a yacht in the Indian Ocean were captured by pirates, who say they will only be freed if a ransom is paid.
Pirates are also threatening to kill three Spanish sailors if two pirates held by the Spanish authorities are not freed.
Somalia has not a functioning national government for 18 years and the lawlessness has spread to the high seas in recent years.