By Stefano Ambrogi

LONDON, July 10 (Reuters) - Two United Nations agencies called for urgent action to combat ship piracy off Somalia on Tuesday, which they said was endangering vital aid shipments and threatening international trade routes close by.

The U.N. International Maritime Organization (IMO), and the U.N. World Food Programme (WFP), said attacks were rising and growing more audacious. The agencies urged the Security Council, the African Union and neighbouring countries to help.

WFP Executive Director Josette Sheeran said crucial supply lines used to distribute aid to a million Somalis were at risk with "potentially disastrous" effects.

"Close to 80 percent of WFP's assistance to Somalia is shipped by sea but, because of piracy, we have seen the availability of ships willing to carry food to the country cut by half," Sheeran said at a joint press conference in London.

She said aid was being funnelled in by other means and had not been severely hit yet but warned the situation could change drastically if the attacks went on unchecked.

She said the aid effort in Somalia was now one of the "most difficult humanitarian operations in the world", because the attacks were targeting hard won suppliers.

The IMO, the world's top maritime body, said there had been 15 ship hijackings this year. Two of the attacks involved WFP-chartered ships and in one of them a security guard was killed.

IMO Secretary General Efthimios Mitropoulos said the pirates were locally based criminals linked to warlords or militia bent on extorting money.

He said they were "trained fighters in military fatigues" sometimes armed to the teeth with automatic weapons and rocket-propelled grenade launchers.

Mitropoulos said their preferred modus operandi was to launch attacks on speed boats from a "mother ship" in territorial waters and up to 200 nautical miles offshore.

He said the more audacious attacks, further out to sea, were close to major international trade routes linking the Red Sea with the Suez Canal and Gulf countries or sealanes down to the Cape of Good Hope.

Some attacks had been committed under the noses of multinational naval forces patrolling the Red Sea who were not able to pursue the assailants into territorial waters because they risked flouting international law, he said.

Mitropoulos renewed calls for the U.N. Security Council to pressure the transitional government to allow war ships to enter its waters.

"Then we may be in a better position to fight this common scourge," he said.

"Without their consent their own people will suffer."

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