Italian tanker Caylyn (pictured) was seized a day before the Greek Mount Irene was hijacked in the Indian Ocean [EPA]

Somali pirates have seized an oil tanker carrying about $200m worth of crude oil off the coast of Oman, the vessel's Greek operator has said.

The Athens-based shipping company Enesel said on Wednesday that the Mount Irene oil tanker was attacked by the group that took the 25 crew members captive.

Authorities have not been able to communicate with the ship, which has seven Greek nationals, 
17 Filipinos and one Georgian onboard.

The vessel was on its way from the Persian Gulf to the Gulf of Mexico when it was attacked 650km south-east of Muscat, the Omani capital. Somali pirate gangs are known to operate in those waters.

The incident comes a day after pirates took control of an Italian oil tanker in the Indian Ocean, some 1300km from Somalia's coast.

'Widening their scope'

Tanker association INTERTANKO said the hijacking of the large tankers marks a significant shift in piracy in the Indian Ocean and could "strangle" vital shipping lanes.

"If piracy in the Indian Ocean is left unabated, it will strangle [these] crucial shipping lanes with the potential to severely disrupt oil flows to the US and to the rest of the world," Joe Angelo, managing director of INTERTANKO, told Reuters.

Phillip Cable, director and founder of Maritime Asset Security and Training, told Al Jazeera that pirates are widening their scope and are getting better.

"The pirate's modus operandi is to stretch the boundaries of where they are operating. We are seeing attacks as far south as the Mozambique Channel and as far east almost at the coast of India. It is without doubt an intractable problem," Cable said.

Somali pirates have made millions of dollars by capturing cargo vessels in the shipping lanes around the Horn of Africa and holding the ships and crew for ransom. Before the latest incident, pirates were holding 29 vessels along with an estimated 681 hostages.