(CNN) -- A South Korean navy destroyer Tuesday caught up with a South Korean oil tanker that was seized earlier by pirates off the coast of Somalia, official Korean media reported.
The 4,500-ton destroyer was keeping a close watch on the hijacked vessel about 30 miles away, South Korean defense officials told Yonhap news agency.
Pirates captured the South Korean tanker Sunday about 690 miles (1,111 km) north of the Somali coast. News of the attack came as the Combined Maritime Forces -- a cooperative effort by several countries to patrol international waters -- warned that pirates in the Somali basin and Gulf of Aden are venturing farther from home waters to target commercial vessels.
The 300,000-ton supertanker, Samho Dream, was seized while on its way to Louisiana in the United States.
Twenty-four crew members -- five South Koreans and 19 Filipinos -- were aboard.
"Ships operating in Somali waters are often accompanied by private or government escorts because of rampant pirate activity in the area, but the ship did not have any guards on board as it was operating in areas where piracy is not frequent," the Yonhap report said.
Valero Energy Corp., the U.S. company that owns the cargo, did not say how much oil was on the ship. But ships of that size generally carry around 2 million barrels of oil, company spokesman Bill Day said.
At current prices, that cargo could be worth about $170 million.