BAGHDAD, Iraq – Two U.S. soldiers died Sunday when their helicopter crashed and about 250 insurgents were reported killed as Iraqi security forces backed by American aircraft fought with militants camped near the Shiite city of Najaf, an area that had been regarded as one of the most stable in Iraq.

According to Iraqi accounts of the fighting, it was the fiercest battle in the country in months.

A brief U.S. military statement said the two Americans were killed when their helicopter “went down” during operations in support of Iraqi security forces near Najaf at 1:30 p.m. local time. The statement did not say what caused the crash, but witnesses said they saw the helicopter fall from the sky trailing smoke from its tail during gunbattles. Iraqi security officials said it had been shot down.

The fighting continued late Sunday night, and Iraqi army officials said they had asked for U.S. armored reinforcements from neighboring Diwaniyeh province. Najaf residents said they could hear warplanes bombing the area late into the night, attacking what appeared to be a large and previously unknown concentration of well-armed and well-equipped insurgents.

Six members of the Iraqi security forces were killed, and 15 wounded. Three Iraqi soldiers were captured by the militants, and 30 of the insurgents were captured, Iraqi officials said.

The U.S. military also announced the deaths of two other service members, one in an explosion north of Baghdad and another in combat in the western province of Anbar.

Najaf’s governor, Assad Sultan Abu Gulal, told reporters at the provincial government’s headquarters that Iraqi security forces launched a dawn raid after receiving intelligence about a group of militants camped out among orchards of date palms at Al-Zarqa, about 12 miles northeast of Najaf.

The militants were suspected of planning to assassinate top Shiite religious leaders in Najaf, including Grand Ayatollah Ali al-Sistani, and attack Shiite pilgrims heading to the nearby city of Karbala for the Shiite religious festival of Ashoura on Tuesday, he said.

Iraqi security forces, who assumed responsibility for security in the province from the U.S. military last December, encountered unexpectedly stiff resistance from the well-trained group of around 500 fighters equipped with light and medium weapons, including mortars and anti-aircraft missiles, as well as radio communications, the governor said.

Najaf officials said there were a number of foreign fighters, including Afghans, among the captured fighters and initially identified the militants as “Baathists and foreign fighters,” suggesting they were Sunni insurgents.

But there was widespread speculation in Najaf that the militants belonged to a hitherto little-known and ultra-radical Shiite group that has recently been challenging the authority of the established religious clergy in Najaf.

Najaf is almost wholly Shiite and had been regarded as one of the safest Iraqi provinces, making the eruption of fighting all the more unexpected. U.S. forces transferred authority to the Iraqi army there in December, in a measure of their confidence in the province’s relative stability.

Col. Ali Nomas, spokesman for the Najaf security forces, said the militant force could have numbered as many as 1,000 and 2,000, based on intercepts of radio communications between the fighters. He said more than 250 corpses had been recovered, but the number could not be independently confirmed.

The eruption of fighting coincided with heightened security measures elsewhere in the country ahead of the Ashoura commemorations, during which Shiites mourn the death in battle of the 7th century Imam Hussein. Hundreds of thousands of Shiite pilgrims are converging on Karbala for the ceremonies, which were banned during the rule of dictator Saddam Hussein.

In Baghdad, the violence continued despite the tighter security. Mortars struck a girls’ school in the western, Sunni neighborhood of Adil, killing five students attending class, according to The Associated Press.

In the Shiite Sadr City neighborhood, a bomb exploded on a minibus, killing eight, and police reported the discovery of more than 50 corpses dumped around the capital.

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