A large Liberian crude oil tanker was attacked and overtaken by pirates several hundred miles off the coast of Kenya, the U.S. Navy reported Monday.

Navy officials said it appears to be one of the largest merchant vessels ever commandeered by pirates.

The ship, named Sirius Star, is owned by Saudi Arabian-based Saudi Aramco and is manned by a crew of 25.

Pirate attacks have become more prevalent off the coast of Africa in recent years.

Navy Vice Adm. Bill Gortney said in a statement that increased U.S. presence in the region has helped curb piracy, but that the incident aboard the Sirius Star shows pirates are adapting “their tactics and methods of attack.”

“Piracy is an international crime that threatens global commerce. Shipping companies have to understand that naval forces can not be everywhere. Self protection measures are the best way to protect their vessels, their crews, and their cargo,” Gortney said.

The Navy said that of 15 recent pirate attacks in the Gulf of Aden off the eastern coast of Africa, at least 10 involved ships operating outside the International Maritime Organization’s recommended traffic corridor.

In addition, many of those ships failed to employ recommended self-protection measures, making them more vulnerable to successful attacks, according to the Navy

“Most notably, none of the vessels had embarked security teams. Embarked security teams would have prevented these successful attacks,” said Gortney. “Companies don’t think twice about using security guards to protect their valuable facilities ashore. Protecting valuable ships and their crews at sea is no different.”

The area off the coast of Somalia and Kenya being frequented by pirates covers more than 1.1 million square miles. That’s about four times the size of Texas, or the size of the Mediterranean and Red Seas combined.

The Navy said the attack on the Sirius Star shows pirates in the region are determined to continue their assaults on an array of merchant ships despite stepped up efforts to stop them.

A week ago, a British warship skirmished with pirates after an attack on a Danish shipping vessel. Several pirates were killed.