NAIROBI, Aug. 21 (Xinhua) -- Armed Somali pirates have hijackeda Japanese tanker and an Iranian bulk carrier in the Gulf of Aden, a regional maritime official confirmed on Thursday.

    Andrew Mwangura, the coordinator of the East Africa Seafarers Assistance Program (SAP), said the pirates opened fire on the Iranian vessel before boarding it on Wednesday night.

    He said that less than an hour later a Japanese-operated tanker with 19 crew was also attacked and seized around the same location.

    "Somali pirates hijacked two more ships last night within hours of each other, an Iranian ship first and later a Japanese ship. I have not established their names," Mwangura told Xinhua by telephone from Mombasa, east Kenya.

    The latest hijackings came two days after a Malaysian palm oil tanker was seized in the same area by pirates.

    Mwangura said authorities would wait until they had a better idea of the identity of the sailors and the hijackers before making contact with the crew.

    The frequency of piracy attacks has been especially high in the past month. Since late July, Japanese, Nigerian, and Thai ships have all been hijacked by Somali pirates seeking ransoms.

    Piracy has long been a problem in the Gulf of Aden, where one of the world's busiest shipping lanes, connecting the Indian Ocean and the Red Sea, passes by lawless Somalia, which has been without an effective central government since 1991.

    The attackers are usually armed with automatic weapons and rocket-propelled grenades and travel in small, fast speedboats that can be towed more than 100 miles offshore by larger vessels to lie in wait.

    Somalia's coastline is considered one of the world's most dangerous stretches of water because of piracy.