Pirates have anchored a hijacked Saudi oil tanker off the Somali coast, as the spate of hijackings gathered pace with two more ships seized on Tuesday.
Vela International said all 25 crew on the Sirius Star - the biggest tanker ever hijacked - were said to be safe.
The vessel is carrying a cargo of 2m barrels - a quarter of Saudi Arabia's daily output - worth more than $100m.
A cargo ship and a fishing vessel were the latest to join more than 90 vessels attacked by the pirates this year.
A Hong Kong cargo vessel was attacked early on Tuesday morning in the Gulf of Aden, the International Maritime Bureau (IMB) said. Chinese media said the ship, with a crew of 25, was carrying wheat intended for Iran.
The other vessel, a fishing boat registered in Kiribati, was carrying a crew of 12, the IMB said. Its owners lost contact with it on Tuesday morning.
The owner of the Sirius Star, seized on Saturday 450 nautical miles (830km) off the Kenyan coast, said they were not aware of any ransom demand, BBC reported.
Vela International said in a statement that it was "awaiting further contact from the pirates in control of the vessel".
The South Korean-built vessel, which is about the length of a US aircraft carrier, was heading for the US via the southern tip of Africa when it was hijacked.
The US Navy described the seizure as an "unprecedented" attack. It confirmed that the tanker had anchored off the Somali coast near the town of Harardhere.
Saudi Foreign Minister Prince Saud Al-Faisal condemned the hijacking as "an outrageous act", but gave no indication of what action, if any, Saudi Arabia intended to take.
Somalis on shore spoke of their surprise at seeing the huge vessel pass just off the coast.
"I headed for the sea to fish, but I saw a very, very large ship anchored less than three miles (5km) off the shore," said Abdinur Haji, a fisherman in Harardhere.
"I have been fishing here for three decades, but I have never seen a ship as big as this one," he said.
Vela International said a response team had been mobilised to work towards ensuring the safe release of vessel and crew.
It said the crew consisted of two British, two Polish, one Croatian, one Saudi and 19 Philippine nationals. The captain is a Pole, Poland's foreign ministry confirmed.
BBC security correspondent Frank Gardner says that the hijacking was highly unusual because of the size of the ship involved and marks a major escalation in piracy.
The seizure points to the inability of a multi-national naval task force sent to the region earlier this year to stop Somali piracy, he says.
Commander Jane Campbell, of the US Navy's 5th Fleet, told the BBC it had warned shipping companies that the US naval presence could "not be everywhere".
"For that reason we have strongly encouraged proactive self-protection measures for the companies," she added.
War-torn Somalia has not had an effective government since 1991 and the anarchy on land has spread to the high seas in recent years.
Hijackings off the coast of East Africa and the Gulf of Aden - an area of more than 1m sq miles - make up one-third of all global piracy incidents this year, according the IMB.
They are usually resolved peacefully through negotiations for ransom.
Fourteen vessels currently remain captive in Somalia, with around 268 crew being held hostage, according to the IMB. One is a Ukrainian freighter seized in September carrying 33 tanks.
Shipping companies are now weighing up the risks of using the short-cut route to Europe via the Suez canal.
"Under normal circumstances we send about 23 ships a week through the Gulf of Aden," Jan Hammer, chief operating officer of Norwegian firm Odfjell, told the BBC.
"Because of the situation with the piracy we have decided that we are no longer - at least until the situation has been rectified - going to send ships through that area, which means that the ships will be routed via the Cape of Good Hope."