Maritime monitoring officials say pirates boarded and hijacked the Iceberg I vessel Monday just 15 kilometers from the port of Aden on the Yemeni coast.
The pirates were reported to be sailing the ship toward Somalia.
On Sunday, Somali pirates demanded a $3 million ransom for a North Korean-flagged cargo ship hijacked early last month.
Andrew Mwangura of the Kenya-based East African Seafarers Assistance Program said the pirates threatened to kill the 10 Syrian crew members on board the MV Rim.
Pirates seized the Libyan-owned ship on February 3 in the Gulf of Aden.
Somali pirates have hijacked dozens of ships over the last few years, taking in tens of millions of dollars in ransom.
Seychelles fishing vessel and an Iranian boat that were also taken in the waters off east Africa were later freed. These released vessels are not among the seven ships cited by the traders.
Sea gangs have acquired millions of dollars in ransoms and defied the presence of highly armed foreign warships that are trying to monitor the region's busy sea lanes.