Somali pirates who seized a Ukrainian freighter carrying Russian-made tanks are threatening a fight to the death as U.S. ships and aircraft surround them and a Russian warship heads to join the scene.

A U.S. destroyer and submarine, as well as other foreign vessels, are surrounding the Faina, which was carrying 33 Russian-built T-72 tanks when it was seized by the pirates on Thursday.

The guided missile destroyer USS Howard anchored in the area Sunday, making sure that the pirates did not remove the tanks, ammunition and other heavy weapons from the ship, which was anchored off the coast. On Monday, more U.S. naval ships and several helicopters arrived off the Horn of Africa.

Russian warship the Neustrashimy, or Intrepid, is also en route to the area, the AFP reported.

The pirates have demanded a $20 million ransom.

"What we are awaiting eagerly is the $20 million — nothing less, nothing more," one of the pirates, Sugule Ali, told a local news agency by satellite phone. "We are not afraid of their presence; that will not make us abandon the ship or refrain from asking the money."

Ali added that his crew of pirates were "not afraid" of the foreign warships.

Original intelligence reports indicated the cargo — which includes Russian tanks, RPGs, and ammunition — was headed for Sudan, but recent declarations by the Kenyan government said the military hardware was intended for them, a U.S. Navy official told FOX News.

He said the mission now is to make sure this military hardware does not end up in the wrong hands.

"It's the type of ammo that could be 'repurposed' and used in ways other than those for which it was originally designed," the official said.

The U.S. fears the armaments onboard the Ukrainian vessel may end up with Al Qaeda-linked Islamic insurgents who have been fighting the shaky U.N.-backed Somali transitional government since late 2006.

"We maintain a vigilant watch over the ship and we will remain on station while negotiations between the pirates and the shipping company are going on," Christensen told The Associated Press.

One crew member died in the hijacking of the vessel, a death Ali said was "natural," and not of gunshots and violence, Australia's Herald Sun newspaper reported.

Christensen did not specify whether the arms were intended for the Khartoum-based Sudanese government, or southern Sudan, which was granted a degree of autonomy under a 2005 peace deal that also guaranteed the oil-rich region a referendum on full independence in 2011.

The U.N. has imposed an arms embargo on weapons headed to Sudan's Darfur conflict zone, but the ban does not cover other weapons sales to the Khartoum government or the southern Sudan's autonomous government.

Kenyan officials on Monday declined to discuss the destination of the weapons. Ukrainian Defense Ministry spokesman Valentyn Mandriyevsky said the ministry was not dealing in weapons trade and didn't know where the cargo was bound.

A spokesman for Ukraine's arms trader, Ukrspetexport, had no immediate comment.

Western intelligence reports a few days ago said the ultimate destination was Sudan and that Kenya was only the transshipment point, said one Western official, who spoke on condition of anonymity because he was discussing classified material. He said the issue became confused after Kenyan leaders had publicly referred to the tanks as their own.

Christensen said an unspecified number of destroyers and cruisers have joined the San Diego-based USS destroyer Howard within a 10-mile radius of the Faina.

"The safety of the ship's crew and cargo is a paramount concern to us," Christensen said, adding additional warships and helicopters were deployed to prevent the weapons from falling "into the wrong hands."

There have been 24 reported attacks in Somalia this year, according to the International Maritime Bureau's piracy reporting center. Last year, U.S. naval helicopters fired on pirate skiffs tied to a hijacked Japanese tanker carrying 30,000 tons of benzene after they feared that pirates might try to use it as a floating bomb in a middle eastern oil port.

Seizing ships has become an important source of income for pirates in Somalia, which is riven between rival clan-based warlords since they overthrew a socialist dictator in 1991.

Click here to read more on this story from the Herald Sun.

Click here for more on this story from the AFP.

FOX News' Justin Fishel, Jennifer Griffin and The Associated Press contributed to this report.