Story highlights Six U.S. Marines die in a helicopter crash, a U.S. military official says

French troops are killed by an Afghan soldier were unarmed at the time, an official says

President Nicolas Sarkozy: France may bring troops home early if security isn't restored

NATO chief Rasmussen pays tribute to four French soldiers killed in Afghanistan

Allied forces suffered a day of heavy losses in Afghanistan Friday after a helicopter crash killed six U.S. Marines and an attack killed four French soldiers, prompting Paris to consider an early troop withdrawal.

The Marines died after their CH-53 helicopter crashed in Helmand province, a U.S. military official said. The NATO-led force reported no enemy activity in the area, but the Taliban claimed credit for bringing down the chopper.

Separately, an Afghan soldier killed four French soldiers and injured 15 others, one critically, in eastern Afghanistan, French officials said. President Nicolas Sarkozy said he was suspending French training operations and combat help as a result.

"The French army is not in Afghanistan to be shot at by Afghan soldiers," he said.

France could bring its troops back early from Afghanistan if the necessary security is not restored, Sarkozy said. France has 3,935 troops in Afghanistan, according NATO's International Security Assistance Force.

Sarkozy will send French Defense Minister Gerard Longuet to Afghanistan, he said.

"It was during a training session inside the base that a shooter killed four of our soldiers. This is unacceptable and demands a full investigation," Longuet said.

A French official told CNN the French troops were unarmed as they were inside their base, conducting normal training operations with their Afghan partners.

The official, who was not authorized to speak to the media, said 15 soldiers were injured.

The attack in Kapisa province follows a similar shooting last month by an Afghan soldier that killed two French soldiers serving in an engineers' regiment, also in eastern Afghanistan.

Friday's suspected shooter, who was a member of the Afghan National Army, has been apprehended, according to an ISAF statement.

Afghan President Hamid Karzai said he was deeply saddened by incident.

"France has been generous to provide extensive assistance to Afghanistan over the past 10 years," Karzai said. "Throughout history, the two countries have enjoyed a sincere relationship, which the Afghan people will always cherish."

Taliban spokesman Zabiullah Mujahid called the attack "sensible."

"This was the latest attack by those sensible and zealous Afghans who have entered the enemy's army and it was also the best one so far as it killed more soldiers than any other such attacks before,' Mujahid said.

Responding to last month's shooting, Sarkozy stressed his country's "determination to continue working within the International Security Assistance Force to restore peace and stability in that country and contribute to its development."

NATO Secretary General Anders Fogh Rasmussen paid tribute to those killed and injured Friday, saying it was a "very sad day" for ISAF troops in Afghanistan and for France.

"Such tragic incidents are terrible and grab headlines, but they are isolated," he said. "The reality is that every day, 130,000 ISAF troops from 50 nations fight and train with over 300,000 Afghan soldiers. That takes a lot of trust among a lot of soldiers.

"We have the same goal. An Afghanistan that is responsible for its own security. That is what Afghans want. And we remain committed to helping Afghans."

ISAF spokesman Lt. Col. Jimmie Cummings also played down the threat, saying, "We train and are partnered with Afghan personnel every day, and we are not seeing any issues or concerns with our relationships."

Friday's shooting was the latest in a series of attacks on NATO forces by members of the Afghan army.

In October, a gunman wearing an Afghan army uniform turned his weapon on coalition forces during training, killing three and wounding several others. The shooter was killed in the incident in southern Afghanistan.

Meanwhile, the cause of the helicopter crash, which happened around midnight Kabul time, is still under investigation, ISAF said.

There was no enemy activity in the area when the helicopter went down in Helmand, said Col. Gary Kolb, an ISAF spokesman.

The Taliban claimed responsibility Friday for bringing down the helicopter.

Qari Yousuf Ahmadi, a Taliban spokesman, told CNN via text message: "A Chinook was shot down in Zubair Karez area, between Musa Qala and Zamin Dawar in southern Helmand province, and a number of foreigners traveling in it were killed."

NATO is scheduled to withdraw its forces from Afghanistan by the end of 2014.

Many ISAF members are involved in missions to train and support Afghan forces as they prepare to take on greater security responsibilities.