Nairobi  - Three warships are hemming in Somali pirates who hijacked a Ukrainian cargo ship carrying tanks and other military supplies, a US Navy official said Monday.

"There are now three ships in the vicinity," Lieutenant Nathan Christensen, Deputy Spokesman for the US Navy's fifth fleet, told Deutsche Presse Agentur dpa.

The MV Faina, along with its cargo of 33 T-72 tanks, armoured personnel carriers and munitions, was seized late Thursday off Somalia as it headed for the Kenyan port of Mombasa.

The USS Howard was the first warship to make visual contact with the seized vessel, which is anchored off the Somali coast near the port of Hobyo.

Two other ships joined the Howard on Monday. Christensen refused to give details about the other warships, but one is believed to be Russian.

The identity of the third is unknown.

Christensen said that there were currently no plans to launch any kind of assault on the hijacked vessel.

"We are maintaining visual contact for the moment," he said.

A pirate spokesman earlier warned that neither the US, nor France - which has troops in nearby Djibouti and has intervened in the past when its own citizens have been seized by Somali pirates - should attempt military action.

"We warn the French and the Americans ... anything that happens is their responsibility," Mogadishu-based Radio Garowe quoted the spokesman, Januna Ali Jama, as saying.

A total of 21 crew members were onboard - 17 Ukrainians, three Russians and one Latvian - when the ship was chased down and boarded by armed men in three launches.

One of the Russian crew members has since died of an illness.

Piracy is rife in the Gulf of Aden - a strategic shipping route off Somalia - with over a dozen ships currently in the hands of armed groups, the latest victim being a Greek vessel seized Saturday.

Two other pirated vessels, MV Capt Stefanos and MV Centauri, are also anchored in the same location as the Ukrainian ship, the US Navy said.

However, pirates have over the past few days released two ships - Japanese vessel the MV Stella Maris and Malaysian tanker the MT Bunga Melati 5 - although ransoms of several million dollars are believed to have been paid.

The ransom being asked for the Belize-flagged Ukrainian ship far exceeds these demands due to its worrying cargo.

The pirates were originally reported to have asked for 35 million dollars, but this figure has now come down, although varying reports put the asking price at between 5 and
20 million dollars.

There has also been confusion over the final destination of the military cargo.

Earlier reports had suggested the shipment was bound for South Sudan, but Kenyan government spokesman Alfred Mutua insists the equipment was for use by the Kenyan military.

South Sudanese officials have denied any link to the tanks.

Piracy has flourished off Somalia the last two years as Islamist insurgents push hard to regain power from the transitional federal government, which came to power with Ethiopian assistance in late 2006.

The United Nations Security Council in June approved incursions into Somali waters to combat the pirates and the US Naval Central Command recently set up a security patrol in the area.

However, the measures appear to have had little effect so far, with estimates putting the number of pirates at well over 1,000, compared to only a few hundred in 2005.

Somali authorities and maritime officials say that paying ransoms has only encouraged more pirates to take to the seas.

Observers have expressed concern that some of the pirates may have links to the ongoing bloody insurgency in the Horn of Africa nation and are helping fund it through piracy.

Almost daily battles have blighted Somalia since Ethiopian troops invaded in 2006 to kick out the Islamist regime and put the transitional federal government back in power.

Somalia has been plagued by chaos and clan-based civil war since dictator Mohamed Siad Barre was toppled in 1991. (dpa)