Somali pirates released a German-operated chemical tanker but others seized another vessel carrying a cargo of petroleum, maritime sources said on Tuesday.
The Marshall Islands-flagged Marida Marguerite and its crew of 22 was hijacked in May by pirates firing automatic weapons and rocket-propelled grenades 120 miles south of Oman. It was freed after its hijackers receiving a ransom.
Andrew Mwangura, head of the Kenya-based East African Seafarers Assistance Programme, said the Marida Marguerite was sailing to safe waters.
Mwangura said later Somali pirates seized the Antigua and Barbuda-flagged Ems River in the Gulf of Aden while it was heading towards the Suez Canal.
The Ems River is a 5,200-dwt cargo vessel, also owned by a German company.
"The ship was taken on Monday, and has about eight crew," Mwangura said.
The European Union naval force, which patrols the Indian Ocean to help crack down on piracy, confirmed on its website the vessel had a crew of one Romanian and seven Filipinos.
The EU force said the Ems River was on its way to Greece from Jebel Ali in the United Arab Emirates at the time of the attack.
"MV Ems River was pirated approximately 175 nautical miles northeast of the port of Salalah, Oman," the EU force said.
It said there were 26 vessels and 609 hostages being held by pirates after being hijacked off the coast of Somalia.
A ransom of about $5.5 million was paid on Sunday for the release of the Marida Marguerite, seized while on its way from Kandla in Gujarat, India, to the Belgian port of Antwerp with a crew of 19 Indians, two Bangladeshis and one Ukrainian, Mwangura said.
He said the ship was probably heading for its original destination of Antwerp.
Pirates are making tens of millions of dollars in ransoms from seizing merchant ships in the Indian Ocean and the Gulf of Aden, despite efforts by foreign navies to crack down on such attacks.
The hijackings have driven up insurance premiums and forced ships to take longer, costlier routes to avoid piracy hot spots.
Industry officials say marine insurers in London's insurance market have widened the stretch of waterways deemed at high risk from Somali pirates as the armed gangs strike further out at sea.
Somalia has been without a functioning government since 1991, when rival warlords overthrew dictator Mohamed Siad Barre and turned on each other. Gangs of pirates emerged from the ensuing chaos to threaten shipping in the Indian Ocean.