Somali pirates armed with rocket-propelled grenades and automatic weapons attacked a Dutch-operated container ship off the coast of Tanzania but failed to hijack the vessel, a maritime official said Sunday.

The attack Saturday shows the pirates are becoming bolder and extending their reach further from their base in Somalia, said Noel Choong, who heads the International Maritime Bureau's piracy reporting center in Kuala Lumpur.

Eight pirates in two speedboats chased and opened fire at the ship about 520 miles east of Dar es Salaam in Tanzania, starting a fire on the vessel that was quickly doused by the crew, he said.

"The pirates used RPGs and automatic weapons to attack the ship but it managed to escape after the captain increased vessel speed," Choong said.

The ship, which has 19 crew members, flies a Hong Kong flag but is operated out of the Netherlands, he said.

Choong said a white-hulled fishing vessel was spotted nearby and was suspected to be the pirates' mother vessel.

The International Maritime Bureau issued an alert to all ships off the coast of Tanzania to be on a strict anti-piracy watch, he added.

Warships from Denmark, India, Malaysia, Russia, the U.S. and NATO patrol a vast international maritime corridor in the Gulf of Aden near Somalia, escorting some merchant ships and responding to distress calls. But the pirate attacks have not abated.

Some 100 ships have been attacked off the Somali coast this year, of which 40 vessels have been hijacked. Thirteen ships remain in the hands of pirates, including a Saudi supertanker filled with $100 million of crude and a Ukrainian ship loaded with 33 battle tanks.

Choong said his bureau has also warned ships to be on alert for suspected Indonesian pirates in the South China Sea off Malaysia's coast. The group is believed to be responsible for four attacks in recent months, he said.

Since September, pirates armed with guns and machetes have robbed three vessels and hijacked a tug and barge off Tioman island, he said.