Pirates hijacked a Philippines chemical tanker with 23 crew near Somalia, bringing the total number of attacks in waters off the impoverished African nation this year to 83, a maritime official said Tuesday.

The tanker was heading to Asia when it was seized Monday in the Gulf of Aden by pirates armed with automatic weapons and rocket-propelled grenades, said Noel Choong, who heads the International Maritime Bureau's piracy reporting center in Kuala Lumpur.

In Manila, Foreign Ministry spokesman Claro Cristobal said the Philippine Embassy in Nairobi and the ship's operator identified the chemical tanker as the MT Stolt Strength.

All 23 seamen on board are Filipino and are "reportedly unharmed," he said, adding that Philippine authorities are coordinating with the ship's operator to secure the early safe release of the vessel and crew.

Choong said there was an attempted attack the same day on a refrigerated cargo ship in eastern Somalia, but the vessel managed to escape with evasive maneuvering. The ship flies a Saudi flag but is operated out of Britain.

Separately, the Indian navy said its marine commandos operating from a warship prevented pirates from hijacking an Indian merchant vessel in the Gulf of Aden on Tuesday.

Choong said the bureau was still verifying the attack with the Indian ship owner. He said there were several other attempted attacks Tuesday, but details are still being ascertained.

The bureau has issued an urgent warning to ships to take extra measures to deter pirates even while sailing in a corridor of the gulf patrolled by a multinational naval force.

"The corridor is protected, but safe passage is not 100 percent guaranteed. The patrol boats cannot be everywhere at the same time. The ship master must maintain a strict radar watch for pirates," he said.

Many ships have fended off pirate attacks after seeking help from the coalition forces, he added.

Russia began escorting a Danish-operated cargo ship with Russian crew members on Tuesday following pirate attacks that claimed another ship operated by the same company last week, officials from both countries said.

NATO has sent three ships to the Gulf of Aden — one of the world's busiest shipping lanes — to help the U.S. Navy in anti-piracy patrols and to escort cargo vessels.

The European Union has said at least four warships backed by aircraft will begin policing the dangerous waters in December. The EU flotilla will eventually take over the NATO patrols.

Despite the increased security, attacks have continued unabated off Somalia, which is caught up in an Islamic insurgency and has had no functioning government since 1991.

As of Monday, there have been 83 attacks this year in Somali waters, with 33 ships hijacked. Twelve vessels remain in the hands of pirates along with more than 200 crew, Choong said, most notably a Ukrainian freighter loaded with tanks and weapons seized Sept. 25.