Somali pirates aboard a ship carrying tanks and weapons that they have seized say they are surrounded by at least three foreign warships.
One is the American US destroyer USS Howard, another is from Russia but the nationality of the third is not clear.
But one of the pirates said they were not afraid and had enough food to withstand a siege.
A maritime expert said the ship was carrying "dangerous chemicals" and warned against using force.
Andrew Mwangura, of the Kenya-based Seafarers' Programme, also told Reuters news agency that a military helicopter had flown over pirate speedboats heading to reinforce the Ukrainian-operated ship, the Faina, moored near the town of Hobyo.
"With the helicopter and the Howard watching them, the tactic is clearly to scare the pirates."
Pirate Sugule Ali told the AFP news agency by satellite phone that his group wanted a ransom of $20m (£11m) and were not interested in the weapons.
Earlier, the pirates had demanded $35m.
"It is true we are surrounded by three foreign military vessels and there are some others we can see in the distance," Mr Ali said.
"We are not afraid of their presence, that will not make us to abandon the ship or to refrain from asking for money.
"There is no shortage of food supply and all the crew members are healthy and well, including ours."
He admitted that one of the kidnapped sailors had died, but said this was from natural causes.
Meanwhile, Kenya has insisted that the shipment of 33 72-T tanks on board were destined for its military.
Various sources have suggested that they were really bound for the autonomous government of South Sudan, in possible contravention of a UN arms embargo.
Mr Mwangura said two previous shipments of Ukrainian weapons had already passed through Kenya.
"There have been alarming propaganda by the pirates to media that the weapons are not for the Kenyan military. This is a tactic by the terrorists to try and fend off reprisals against them," Kenya's government spokesman Alfred Mutua said.
The former rebel SPLA which governs South Sudan has denied any links to the tanks, reports the UN-sponsored Radio Miraya FM.
However, it also quoted the SPLA's Major General Byor Ajang as saying that the army had the right to import weapons from anywhere in the world without co-ordination with the government in the north.
Earlier, a spokesman for the US Navy's 5th Fleet, Lt Nathan Christensen, said the USS Howard was within 8km (5 miles) of the Ukrainian vessel, but refused to say whether they were preparing to attack the pirates.
He said the ship's cargo of battle tanks made it a particularly worrying situation.
"We're concerned that this might end up in the wrong hands, such as terrorists or violent extremists," he said.
Islamist insurgents, not known to have links to the pirate gangs, are battling government troops, their Ethiopian allies and African Union peacekeepers in the capital, Mogadishu.
Somalia has been without a functioning central government for 17 years and has suffered continual civil strife, with rival armed groups fighting for control.
The waters off the coast of Somalia are considered some of the world's most dangerous.
Even ships carrying food aid are often targeted, hampering the delivery of humanitarian supplies to the estimated three million Somalis in need of aid.
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.
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.