MANAMA, BAHRAIN, DEC. 25 -- U.S. and British helicopters plucked 20 crewmen from a lumber-laden South Korean freighter in the southern Persian Gulf after Iranian gunboats set it ablaze today, shipping and military officials said.

Eight hours later, Iranian gunboats attacked the 9,566-ton Saudi Arabian tanker Nejmat Petrol off the emirate of Ras Khaymah in the southern gulf near the Strait of Hormuz, gateway to the gulf. Shipping sources said a fire broke out, but they did not know the extent of damage or injuries.

Gunboats attacked the 19,682-ton Hyundai 7 about 20 miles northeast of the emirate of Sharjah, said marine salvage executives, who spoke on condition of anonymity.

A British Defense Ministry spokesman in London said the frigate HMS Scylla heard a distress call sent by the Hyundai 7 and dispatched helicopters to the rescue.

Helicopters from the Scylla rescued nine crewmen from the Hyundai 7, and U.S. Navy helicopters picked up another 11, the Defense Ministry said in a separate statement. The spokesman said two seriously injured men were treated on the Scylla.

Shipping sources said all were South Korean nationals.

The crewmen were flown to the Scylla, where the ministry said they were "enjoying typical Royal Navy hospitality and Christmas dinner."

A Pentagon spokesman in Washington, Lt. Cmdr. Chris Baumann, confirmed that 20 people had been rescued.

Crewmen aboard a helicopter chartered by the CBS television network spotted smoke from the Hyundai 7 about an hour after the attack and flew to investigate, said CBS producer Bruce Dunning, who was aboard the helicopter.

"The superstructure was engulfed in flame and smoke. Obviously they {the attackers} had aimed for the crew compartment and the bridge," Dunning said in a telephone interview from Dubai. He said the ship had several "pretty good-sized" holes in its hull. "We figure it was done by an Iranian frigate, not by the little Boghammers," a reference to the fast Swedish speedboats Iran often uses in attacks on shipping.

The London-based Lloyd's Shipping Intelligence Unit said the fire had been extinguished by nightfall and the freighter was being towed to Dubai by two salvage tugs.

Dunning said the ship was near Abu Musa, an island that has been used as a staging post for Iranian attacks on shipping. The attacks are in response to Iraqi strikes on Iran's oil terminals and tankers farther north in the gulf.

Iraq and Iran have attacked 26 commercial ships so far this month, the highest number of attacks on shipping in a month since the so-called tanker war began in earnest in 1984 as a spillover of the land war.

It was the second time in two weeks that U.S. Navy helicopters rescued crewmen on vessels attacked by Iranians in the southern gulf. Helicopters from the guided-missile destroyer USS Chandler and a CBS-chartered helicopter rescued 40 people on Dec. 12 from the Cypriot tanker Pivot, which also was set ablaze in an Iranian attack.

U.S. Navy officials have said humanitarian help for seamen in distress does not violate the Pentagon's rules of engagement in the gulf, which prohibit direct protection of foreign-flag ships.

But the Pentagon disclosed this week that it has broadened those rules of engagement to extend protection to allied warships in case of attack. The British Defense Ministry spokesman said the British rules of engagement had not changed.

"This was a purely, 100 percent search-and-rescue mission," he said. The "Scylla received a distress signal, and under international law a nation's ships go to the rescue of those in distress at sea."

The U.S. Navy has 11 ships in its Middle East Task Force in the gulf to protect and escort U.S.-flagged Kuwaiti tankers. Britain, the Soviet Union, France, Italy, Belgium and the Netherlands also have warships in the gulf or just outside.