Armed pirates hijacked three ships off the coast of Somalia in a series of attacks Thursday, an international maritime body said, as it urged the U.N. to restore law and order to the notorious African waters.

Three hijackings in a day "is unheard of," said Noel Choong, the head of the Kuala Lumpur-based piracy center of the International Maritime Bureau.

The attacks came two days after the seizure of a Malaysian palm oil tanker with 39 crew in the same area and raised to seven the number of ships hijacked in the Gulf of Aden since July 20.

In the first incident Thursday, pirates "continuously fired" on an Iranian bulk carrier before boarding and commandeering it, Choong told The Associated Press. Less than an hour later, a Japanese-operated tanker with 19 crew was attacked and seized near the same location, he said.

Later in the day, a German-operated cargo ship, flying the Antigua and Barbuda flag, was hijacked in the vicinity, he said. No details about the crew were available.

He said the piracy center in Kuala Lumpur received a distress call about the German hijacking from a passing ship.

He said "the situation is getting very dangerous" in the waters off Somalia, the world's piracy hotspot where 24 attacks were reported in the first half of this year.

"We strongly urge the U.N. and the international community to take serious action to stop this menace," he said. "If nothing is done more ships may be hijacked and there will be serious consequences on this important shipping route."

Choong said there has been no communication so far with any of the four vessels hijacked since Tuesday.

But a multi-coalition naval force in the area has been informed and is "taking action." The naval force includes the United States, France, Germany, Pakistan, Britain and Canada, which currently holds the rotating command.

The IMB also issued an urgent warning to all ships in the Gulf of Aden to maintain a strict watch. The Gulf of Aden connects the Red Sea and the Indian Ocean, and is one of the world's busiest shipping lanes.

Choong said pirates seized a Japanese-owned cargo ship with 20 Filipino sailors on July 20 in the gulf. A Nigerian vessel was later hijacked followed by a Thai cargo ship with 28 crew members earlier this month.

Pirates in all of those cases demanded ransom for the release of the crews, and negotiations are continuing, he said.

The impoverished country of Somalia has not had a functioning government since 1991 and foreign vessels are frequently seized for ransom by pirates armed with rocket-propelled grenade launchers and automatic weapons, making it difficult and expensive to deliver aid to the region.

In June, the U.N. Security Council voted to allow international warships to enter Somali waters to combat the problem. But its 1,880-mile coastline — the longest in Africa — remains virtually unpoliced.