European Union (EU) naval forces operating off the Somali coast have reported a series of successes in their patrols against pirates, arresting 16 suspects after two attacks in the last four days.

The EU's Maritime Security Centre reported Tuesday that a Swedish warship, the HMS Malmo, chased a skiff in the Gulf of Aden after pirates had fired upon a Greek bulk carrier with small arms and a rocket-propelled grenade earlier in the day.

The Malmo fired warning shots and flares, the centre added. "The skiff stopped after pursuit and was boarded by a vessel protection detachment… On board the skiff some weapons, GPS equipment, grappling hooks and barrels of fuel were found. Seven suspected pirates were captured and are at present being held…"

On Friday the centre reported that the Italian warship Maestrale dispatched a helicopter to the scene of attacks by pirates on two different merchant ships in the Gulf of Aden.

"When the… helicopter arrived," the centre said, "it fired warning shots toward the pirates in order to stop the [second] attack." The Maestrale then sent a boarding party, which seized nine suspects.

Earlier last week, a skiff carrying six pirates attacked a tanker flying under the flag of the Marshall Islands in the eastern Gulf of Aden. Under fire from small arms and rocket-propelled grenades, the tanker took evasive action until a naval frigate arrived and stopped the pirates.

The International Maritime Bureau's piracy reporting centre is warning shipping in the Gulf of Aden and off Somalia, Kenya and Tanzania to keep a strict 24-hour visual and radar watch for pirate suspects.

"Watch-keeping crews should look out for small suspicious boats" approaching their vessels, the centre says. "Most of these attacks have taken place during the hours of darkness."