U.S. soldiers guarding the American Embassy in Kabul (search) shot and killed four Afghan soldiers on Wednesday as they unloaded weapons destined for a nearby intelligence depot, apparently mistaking the men for assailants, Afghan officials said.

The shooting came a day after the United States raised its terror alert level, warning of possible attacks on Americans.

"It was a misunderstanding between the American guards at the U.S. Embassy and our soldiers who were unloading weapons," Kabul Police Chief Basir Salangi told The Associated Press.

Salangi said three Afghan soldiers were killed and two wounded. Hospital officials said another soldier died shortly after being brought in.

Gen. Abdul Raouf Taj, chief commander for the Kabul district where the embassy is located, also said four Afghan soldiers had been killed. He said four others were wounded.

There were no apparent U.S. casualties.

Lt. Col. Paul Kolken, a spokeswoman for the international peacekeepers that patrol the capital, said there were unconfirmed reports that the Afghan soldiers shot first, firing at a passing car in front of the U.S. Embassy for unknown reasons.

"In doing so, they fired in the direction of the American embassy and the American soldiers standing guard there returned fire," Kolken said.

He said the first shots were fired just after 10 a.m., adding the incident was still being investigated.

Tension has been high in the Afghan capital in recent months. There was an explosion not far from the U.S. Embassy in April, and a rocket slammed into the nearby headquarters of international peacekeepers on March 30. Neither attack caused any casualties.

U.S. embassies around the world are on heightened alert following a decision by Washington to raise the terrorism alert to orange, signifying a "high" risk of attacks.

Also Wednesday, President Bush's special envoy to Afghanistan, Zalmay Khalilzad (search), was due to visit Afghanistan. It was not clear if he was in the capital at the time of the shooting.

Khalilzad's convoy was seen leaving the embassy about two hours after the shooting, heading in the direction of the presidential palace at the end of the street.

International peacekeepers in armored trucks mounted with machine guns tried to block off the area around the embassy following the shooting, with Afghan military vehicles blocking streets leading to the U.S. compound.

The heavily policed road where the shooting occurred is also home to the international peacekeeper's base, an Afghan intelligence agency barracks and leads to the Presidential Palace.

Salangi said the soldiers were delivering the weapons to the intelligence agency across from the embassy.

"They wanted to unload the weapons to store them inside the brigade, which belongs to the intelligence service," he said.

U.S. embassy and Afghan government officials had no immediate comment.