Pirates who seized a Ukrainian ship off the coast of Somalia have warned against any attempt to rescue the vessel's crew or cargo, reported BBC.
One of the pirates said they would speak to Kenyan authorities about a ransom for the ship, which is carrying 33 battle tanks destined for Kenya.
Pirates have seized dozens of ships near Somalia's coast in recent months.
A Russian Navy vessel is heading to the region and the US says it is monitoring developments in the area.
Ukrainian Defence Minister Yury Yekhanurov earlier confirmed 33 Russian T-72 tanks and "a substantial quantity of ammunition" were aboard the captured cargo ship, called the Faina.
Ukraine's foreign ministry said the ship had a crew of 21 and was sailing towards the Kenyan port of Mombasa.
The ship's captain had reported being surrounded by three boats of armed men on Thursday afternoon, it said.
Earlier reports suggested that the cargo had been destined for south Sudan, but Kenyan government spokesman Alfred Mutua confirmed the tanks were heading to Kenya.
"The cargo in the ship includes military hardware such as tanks and an assortment of spare parts for use by different branches of the Kenyan military," he said.
Security analyst Knox Chitiyo told the BBC the latest incident showed the waters off Somalia's coast had become a "global security problem".
"Piracy has become big business and there seems to be no concerted response to the problem," said Mr Chitiyo, from the London-based Royal United Services Institute.
Authorities in Somalia's semi-autonomous region of Puntland say they are powerless to confront the pirates, who regularly hold ships for ransom at the port of Eyl.
Senior UN officials estimate the ransoms pirates earn from hijacking ships exceed $100m (£54m) a year.
Pirate "mother ships" travel far out to sea and launch smaller boats to attack passing vessels, sometimes using rocket-propelled grenades (RPGs).
Last week, France circulated a draft UN resolution urging states to deploy naval vessels and aircraft to combat such piracy.
France, which has troops in nearby Djibouti and also participates in a multi-national naval force patrol in the area, has intervened twice to release French sailors kidnapped by pirates.
Commandos freed two people whose boat was hijacked in the Gulf of Aden earlier this month and in April, six arrested pirates were handed over to the French authorities for trial.
Russia announced on Friday it would start carrying out regular anti-piracy patrols in the waters off Somalia to protect Russian citizens and ships. A warship had been sent to the area earlier this week, it said.
Somalia has been without a functioning central government for 17 years and has suffered from continual civil strife.