A US navy destroyer has made visual contact with a Ukrainian ship which was seized by pirates last week and is now moored off the Somali coast.
There is no indication that the USS Howard is about to approach the MV Faina, which is carrying 33 T-72 battle tanks destined for Kenya's government.
A Russian navy vessel is also heading towards the region.
The pirates have reportedly demanded a ransom of $35m (Ј19m) to release the Ukrainian vessel and its crew.
But the Kenyan government has cast doubt on the report, saying it had not been issued with ransom demands.
The pirates also warned against any attempt to rescue the crew or cargo of the ship.
International concern
In an interview with the BBC, one of the pirates, Januna Ali Jama, claiming to be speaking on their behalf, said they were prepared to negotiate with the Kenyan government, but that would not release the MV Faina unless the ransom was paid.
"We are warning France and others who are thinking of carrying out a rescue that we have the power to reach them wherever they are," he said.
"We are demanding a ransom of at least $35m."
Later, in a tersely-worded statement, a spokesman for the Kenyan government said they had not received any credible demands for a ransom to release the ship.
He went on to say that the government would not negotiate with what it called international criminals, pirates and terrorists, and said efforts to recover the hijacked ship and its cargo would continue.
The BBC's Karen Allen, in Nairobi, says there is constant monitoring of the international waters off the coast of Somalia - considered some of the world's most dangerous waters.
'Big business'
On Friday, Ukrainian Defence Minister Yury Yekhanurov confirmed that 33 Russian T-72 tanks and "a substantial quantity of ammunition" were aboard the MV Faina.
Ukraine's foreign ministry said the ship had a crew of 21 and was sailing towards the Kenyan port of Mombasa.
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 (Ј54m) a year.
Pirate "mother ships" travel far out to sea and launch smaller boats to attack passing vessels, sometimes using rocket-propelled grenades.
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.
Somalia has been without a functioning central government for 17 years and has suffered from continual civil strife.
(BBC)
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