U.S. copter crashes in Afghanistan; 6 dead KANDAHAR, Afghanistan - A NATO helicopter has crashed in southern Afghanistan, killing six members of the international military force, the U.S.-led coalition said Friday. A source with the NATO force in Afghanistan tells CBS News it was a U.S. Marine helicopter which crashed and all those aboard were believed to be Marines, but the coalition did not officially disclose the nationalities of those killed. Details of such incidents are generally kept private until the families of the dead are notified. CBS News national security correspondent David Martin reports it was a CH-53 helicopter that went down in Helmand province. The cause is still being investigated, but a coalition statement said there was no enemy activity in the area at the time of Thursday's crash, which brought the number of international forces killed in Afghanistan this month to 24. The Taliban claimed responsibility for bringing the helicopter down in a statement to CBS News, but the militant group often exaggerates or claims attacks which are later confirmed to have been accidents. The helicopter crash occurred on the same day that a suicide car bomber killed at least seven civilians outside a crowded gate at Kandahar Air Field, a sprawling base for U.S. and NATO operations in the south. The Taliban insurgents claimed responsibility, saying they were targeting a NATO convoy. The coalition said no NATO troops were killed Thursday. It does not disclose information about wounded troops. The Taliban have been stepping up attacks in southern Afghanistan, the birthplace of the insurgency, with a wave of bombings and the assassinations of three local Afghan officials this week. The violence comes even as the U.S. is moving ahead with plans for negotiating with the Taliban to try to end the 10-year-old war in Afghanistan. Two witnesses told The Associated Press that they suspect Thursday's suicide car bomber was trying to hit U.S. troops 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 that NATO forces opened fire after the bombing and that they killed three of the seven civilians who died. 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. On Wednesday, 13 civilians, including three Afghan policemen, were killed when a suicide attacker blew himself up in a bazaar in neighboring Helmand province. Several current and former U.S. officials said the most substantive give-and-take to date between U.S. and Taliban negotiators could happen in the next week, with the goal of establishing what the U.S. calls confidence-building measures - specific steps that both sides agree to take ahead of formal talks. However, U.S. intelligence agencies recently offered a gloomy prognosis in their latest Afghanistan report. The Afghan National Intelligence Estimate warns that the Taliban will grow stronger, using the talks to gain credibility and run out the clock until U.S. troops leave Afghanistan, while continuing to fight for more territory, say U.S. officials who have read the classified document. They spoke on condition of anonymity to describe the roughly 100-page review, an amalgam of the intelligence community's predictions of possible scenarios for the Afghan war through the planned end to U.S. combat in 2014. The report says the Afghan government has largely failed to prove itself to its people and will likely continue to weaken and find influence only in the cities. It predicts that the Taliban and warlords will largely control the countryside.