(CNN) -- The hijacking of a container ship in the Indian Ocean Friday afternoon is evidence of a "constantly expanding area of pirate activity" off the Africa coast, military officials said.

At least five pirates, using two small boats and a rocket-propelled grenade, boarded the MV Panama, a Liberian-flagged vessel operated by a U.S. company, according to a statement by the European Union Naval Force Somalia.

The attack happened about 80 nautical miles east of the Tanzanian-Mozambique border, the statement said.

"This extreme southerly attack in the Somali Basin is a further example of the constantly expanding area of pirate activity," it said.

Pirate attacks have become a frequent hazard for sailors off Somalia, which has had no effective central government since 1991.

The merchant vessel was sailing from Dar es Salaam, Tanzania, to Beira, Mozambique, when the pirates attacked, it said.

There was not immediate word of the condition of the 23 crew members, who are all from Myanmar, it said.

The European Union Naval Force Somalia is a multinational task force with the mission to escort merchant vessels carrying humanitarian aid of the World Food Program and vessels of the African Union Mission in Somalia.

The mission is "to protect vulnerable ships in the Gulf of Aden and Indian Ocean and to deter and disrupt piracy," it said.

Despite a crackdown by an international naval flotilla in the region, pirates managed to seize 35 ships in the region between January and September, according to the International Maritime Bureau, which monitors attacks.