Kenya ? Striking into the heavily patrolled Gulf of Aden, Somali pirates seized a British-flagged chemical tanker — the first merchant vessel to be hijacked there in nearly six months, the same day that a ship was taken by brigands in the Indian Ocean, officials said Tuesday.

The double hijacking late Monday shows that, a year after an international naval armada began deploying off Somalia to protect shipping, piracy remains a problem. Monday’s attacks occurred more than 1,000 miles apart, indicating the wide range of territory prowled by pirates and underscoring the difficulty of policing such a large area.

Cmdr. John Harbour, the spokesman for the European Union’s anti-piracy force, said the seizures were likely only a coincidence and not coordinated because several pirate bands operate in the Indian Ocean and Gulf of Aden.