Somali forces raided one of the many ships hijacked off the country's coast Sunday as a deadline loomed in a standoff aboard another, arms-laden vessel, officials said.

Troops in northern Somalia's semiautonomous Puntland region unsuccessfully tried to take back a ship that was hijacked by pirates on Thursday, said Ali Abdi Aware, Puntland's foreign minister. He said two pirates were killed.

The vessel, which was carrying cement, is believed to have Syrian and Somali crew on board.

"Our forces are chasing the ship and we hope to rescue it," Aware said in a telephone interview from Puntland, a hotbed of piracy.

Meanwhile, pirates on the Ukrainian MV Faina, which is carrying 33 tanks and other heavy weapons, continued to demand ransom money before releasing the ship and its 20 crew.

The pirates have threatened to destroy the vessel Monday night or early Tuesday unless the shipowners pay a ransom of up to $20 million. They have held the ship for more than two weeks.

Sugule Ali, a spokesman for the pirates, said by satellite telephone that negotiations with the shipping company were continuing. Regarding the ransom, he said: "It is before Tuesday or never."

Pirates have seized more than two dozen ships this year off the Horn of Africa, but the Faina has drawn the most international attention because of its dangerous cargo. Many fear the weapons on board could end up in the hands of Islamic militants in Somalia.

The ship's operator, Tomex Corp. in Odessa, has not commented on negotiations.

The threat by the pirates on the Faina was unusual. Pirates operating off Somalia rarely harm their hostages, instead holding out for a ransom that often exceeds $1 million.

But international pressure on the pirates is growing. NATO said Thursday it would send seven ships to the treacherous waters where pirates are negotiating the release of the Faina. U.S. warships are surrounding the ship, and a Russian vessel is on the way as well.

There are 20 Ukrainian, Latvian and Russian crew members on board.

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The ship's Russian captain died of a heart condition soon after the hijacking nearly two weeks ago, officials in Moscow say.

Lt. Stephanie Murdock, a spokeswoman for the 5th Fleet, which is based in Bahrain and helps monitor Somalia's coast, said there were no significant developments Sunday.

A nation of around 8 million people, Somalia has not had a functioning government since 1991. A quarter of Somali children die before age 5 and nearly every public institution has collapsed. In the capital, Mogadishu, thousands of civilians have died over the past 18 months in a ferocious, Iraq-style insurgency.