Updated 9:01 PM ET
KANDAHAR, Afghanistan - A NATO helicopter crashed in southern Afghanistan on Thursday, killing six members of the international military force, the U.S.-led coalition said.
The coalition said in a release early Friday morning that there was no enemy activity in the area at the time of the crash in southern Afghanistan.
CBS News national security correspondent David Martin reported that a CH-53 helicopter went down in Helmand province.
The cause of the crash is still being investigated. The coalition did not disclose the nationalities of those killed.
The helicopter crash occurred on the same day seven civilians were killed outside a crowded gate at Kandahar Air Field, a sprawling base for U.S. and NATO operations, after a suicide attacker set off a vehicle laden with explosives. The Taliban claimed responsibility, claiming they were targeting a NATO convoy.
It was the second suicide bombing in as many days in southern Afghanistan, officials said. The coalition said no NATO troops were killed. It does not disclose information about injured troops.
Separately, Afghan authorities reported Thursday that avalanches have killed at least 29 people in the country's mountainous northeast.
Two witnesses told The Associated Press that they suspect the suicide car bomber was trying to hit U.S. forces because he detonated his explosives just as two pickup trucks, which they say are often used by American special forces, were leaving the base.
Taliban spokesman Qari Yousef said NATO forces opened fire after the bombing, and that they killed three of the seven civilians who died. The coalition denied this, saying there was no fighting after the blast.
Earlier, officials reported that the suicide bomber was walking near the gate, but the Afghan Ministry of Interior later said the attacker was driving a Toyota Corolla.
Zalmai Ayubi, the spokesman for the Kandahar provincial governor, said two children were among the seven civilians killed. He said eight other civilians, including two children and one woman, were injured in the explosion.
Gates to the larger U.S. bases in Afghanistan often are crowded with trucks waiting to deliver goods and services, and local Afghans going to or coming back from jobs on the compounds.
Safiullah, a 40-year-old fuel tank driver from neighboring Zabul province, was waiting his turn to enter the base when the blast occurred.
"There was dust and smoke everywhere," said Safiullah, who uses just one name. "I got down on my knees. When the smoke lifted, I moved closer. I saw two children dead at the side of the road."
At the time of the explosion, two pickup trucks were leaving the base, he said. He said he remembered that because he and another man were conversing at the time about how U.S. special forces sometimes use that kind of truck.
The explosion shattered the window of a taxi driven by Sabiullah Khan, who was at the gate waiting for customers.
"I put my head down in my car," he said. "For three or four minutes I was afraid. I was reciting the words of the Quran," the Muslim holy book. "When the smoke cleared and I knew I was OK, I started looking outside. People were shouting for help. I saw one vehicle on fire. The Afghan army were running and taking out the wounded."
He said that when he got out of his taxi, he also saw the two pickup trucks.
"Nobody was in them, but from the condition of the vehicle, I'm sure that if they were not killed, they were wounded," he said.
On Wednesday, 13 civilians, including three Afghan policemen, were killed when a suicide attacker blew himself up in a bazaar in neighboring Helmand Province.
The Helmand governor's office said 22 others were wounded in the blast in Kajaki district.
The coalition said some international troops were killed and wounded in the attack, but did not disclose details.
Late Wednesday, NATO reported that one coalition trooper had been killed in an explosion in southern Afghanistan, but would not say whether the service member died in the Kajaki bombing, or some other incident.
U.S. Gen. John Allen, the top commander of American and NATO forces in Afghanistan, condemned the Kajaki attack, saying it was evidence that the Taliban insurgents had "declared outright war" on the Afghan people. He said that such violence "will only further isolate the Taliban from the process of peace negotiation."
The U.S. has been working to broker talks between the Taliban and President Hamid Karzai's government to end the 10-year war. The insurgents recently said they would open a political office in the Gulf state of Qatar to pursue negotiations but would also continue fighting.
In northern Afghanistan, 29 people have died in avalanches reported since Monday in Badakhshan province. according to the Afghan National Disaster Management Agency.
At least 40 more people have been injured and rescuers were struggling to reach areas of Afghanistan's mountainous northeast that have been cut off by heavy snows.
Roads outside the provincial capital of Faizabad were blocked by at least 6 feet (2 meters) of snow, the agency said.
Afghanistan's harsh winters and mountainous terrain in the north make avalanches a danger each year.
In February 2010, an avalanche killed at least 171 people near the Salang Pass, a major route through the Hindu Kush mountains that connects the capital of Kabul to the north of the country.