( CNN ) -- A Yemeni oil tanker hijacked in the Gulf of Aden was freed Monday and 11 Somali pirates were arrested , Yemen 's official news agency said . 11 pirates were arrested by Yemeni security forces in an operation to free the oil tanker . Three marines were wounded in the operation conducted by Yemeni forces , according to the SABA news agency . The tanker was seized Sunday on its way to Aden , SABA said . Aden is a port city in southwestern Yemen . Meanwhile , two pirates were killed and one injured when Yemeni troops freed three other ships Sunday , SABA reported . Four pirates were also arrested in the operation , it added . Piracy has become a major problem in recent years in the waters off Somalia , which has not had an effective government since 1991 . Despite the introduction of international naval patrols in the region , pirates have continued to attack commercial vessels and collect large ransoms , running into millions of dollars , from shipping companies . Attacks on ships in the Gulf of Aden and off Somalia 's coast accounted for 61 of the 102 attacks during the first quarter of this year . That compares with six incidents for the same period in 2008 , said the International Maritime Bureau , which issues regular reports on piracy worldwide . The European Union and several nations , including the United States , have naval forces in the region to protect vessels against pirate attacks . The head of EU naval forces in the waters off Somalia said recently that restoring long-term stability to Somalia will be what ultimately stops the attacks . Rear Adm. Philip Jones said : `` It 'll be a long period of time before that 's successful , and we must be ready to secure the seas until that 's in place . ''
Oil tanker was seized Sunday on its way to Yemeni port of Aden . Three Yemeni marines were wounded , two pirates killed in the operation . Score: 6 Somalia has not had a stable government in place since 1991 .