In the past three days there have been four pirate attacks in the Gulf of Aden, and a Danish war ship has been involved in frightening off the assailants in two of the cases, the Danish navy said Sunday.

"For the second time in a period of 72 hours, the Danish war ship Absalon has gone into action against pirates spreading terror in the Gulf of Aden," the navy said in a statement.

The Danish ship, which is part of an international task force to stem the piracy problem off Somalia's northern coast, had first been called in to assist the Front Voyager trade vessel, located some 160 nautical miles from Absalon's location.

"Due to the distance, Absalon decided to send in its armed helicopter," which arrived on the scene as the crew was desperately using fire hoses to keep pirates from mounting their vessel, the navy said.

The helicopter held the attackers at bay until a US war ship could come to the rescue.

Not long after, the Absalon received another distress call from the Bunga Seroja Sato trade ship, which said it was being tailed by four motor boats that appeared to have pirates onboard.

The Danish ship ordered the trade vessel, located some 20 nautical miles away, to shift course in its direction.

"We consider that Bunga Seroja Sato's changed course and increased speed, as well as Absalon's arrival on the scene, contributed to the suspected pirates giving up and leaving the area," the navy said.

In all, there have been four pirate attacks in the Gulf of Aden in the past three days, the navy said, adding that three of the attacks "have been foiled due to the international coalition force Task Force-150."

The outcome of the fourth attack remained unclear, navy spokesman Jesper Lynge told AFP.

The waters off Somalia, which has lacked an effective government since 1991, are the most dangerous in the world for pirate attacks, with the International Maritime Bureau reporting 24 attacks in the area between April and June this year.

According to Amnesty International, pirates are currently holding more than 130 crew members of ships captured in the Gulf of Aden.