IRBIL, Iraq - In another friendly-fire incident, American planes apparently fired by mistake on allied Kurdish guerrillas and U.S. special-forces soldiers yesterday, killing at least 18 people and injuring more than 45, Kurdish officials said.

U.S. Central Command said its "early casualty reports" on what appeared to be the same incident gave lower figures: one civilian killed and six people injured, including a U.S. soldier. But the command said the investigation was not complete.

The incident showed the heightened intensity of combat along the northern front, where joint U.S.-Kurdish ground offensives backed by air power have been battling Iraqi forces.

In the friendly-fire incident, a translator for the British Broadcasting Corp., Kamran Abdul Razzaq, also was killed.

"Coalition aircraft were conducting close air-support missions at the time, and were in coordination with ground forces," the U.S. Central Command said in a brief statement. "The circumstances contributing to the incident are under investigation."

In a separate incident yesterday, an eight-car convoy of Russian diplomats leaving Baghdad for the Syrian border 200 miles away came under fire, and at least four people were injured, the Foreign Ministry in Moscow said.

Russia did not indicate whether it believed coalition or Iraqi forces were responsible. Central Command said initial field reports indicated that the incident occurred in Iraqi-controlled territory and that no coalition forces were in the area.

A Russian journalist in the convoy said the group, which included the Russian ambassador, was caught in cross fire between coalition and Iraqi forces.

In the Kurdish incident, Wagih Barzani, the commander of Kurdish special forces and the younger brother of Kurdistan Democratic Party leader Massoud Barzani, was critically injured. The party runs half of the Kurdish autonomy zone.

The younger Barzani was flown by helicopter to Bashur airfield, where he was treated and evacuated to a military hospital in Germany. He was in stable but serious condition with a shrapnel injury to the brain, said Lt. Col. Harry Stinger, the commander of the 250th Forward Surgical Team.

The incident occurred near Dibakan, a town recently liberated by Kurdish guerrillas about 30 miles southeast of Mosul, Iraq's third-largest city.

A group of about 18 vehicles in the U.S. and Kurdish convoy, including military transports and journalists' vans, was making its way to Dibakan when the commander of the U.S. team reportedly stopped to call in an air strike. It was believed he had seen an Iraqi tank that had targeted the convoy.

BBC world-affairs editor John Simpson, who was in the convoy, said he saw two U.S. F-14 jets come in low over the convoy. What followed, he said, was "every type of horror."

"I saw the bomb coming out of one of the planes, just one bomb, and then, extraordinarily, I saw it as it came down beside me," he said. "It was painted white and red and it crashed into the ground about 10 yards from where I was standing."

Several of the convoy's vehicles had been carrying ammunition and rockets, which later exploded in the fires caused by the bombing. TV footage of the scene showed a dozen cars and trucks burned and twisted along the road.

Elsewhere on the northern front yesterday, more than 1,000 Kurdish militiamen, aided by U.S. special forces, swept into the town of Ain Sifni on the way toward Mosul and the region's major oil fields.

American air strikes and Kurdish ground attacks have driven Iraqi government forces back from Kurdish territory toward the main strategic prizes of northern Iraq - Mosul and the oil center around Kirkuk.

The Kurds are less than 20 miles from each city.