KABUL  - US troops on Monday reinforced a remote military outpost after well-armed militants got inside and killed nine American soldiers in the deadliest assault on US forces in Afghanistan in three years. It was a "concerted attempt" to overrun the small base near the Pakistan border that was built only about three days ago, said an official with NATO's International Security Assistance Force. The official, who spoke on condition of anonymity because he wasn't authorized to release the information, estimated the attacking force was several hundred. Meanwhile, the Taliban have shot dead at least seven Afghan civilians whom they captured on suspicion of working for the government or other organisations, police and the militants said Monday. The civilians were taken from cars, buses and taxis that were stopped on the main road between Kabul and the southern city of Kandahar on Sunday, said the deputy police chief for Zabul province, Jailani Khan. The police chief of Ghazni, Khan Mohammad Mujahed, said he had reports that 15 people were taken from the road and had all been killed. His men were trying to locate the bodies, he said. Mujahed said the men were dragged from cars along the highway over the past several days. A spokesman for the Taliban, Zabihullah Mujahed, also said the militants had killed 15 "spies of the government" who had been captured on the road in recent days. Taliban militants claimed responsibility Monday for the abduction of an Afghan senator who was snatched at a gunpoint 70 kilometres from capital Kabul. Abdul Wali, a member of the Upper House of the parliament from the province of Logar adjoining Kabul, was kidnapped Sunday as he was driving with his two guards and driver, police said. "Taliban have abducted Dr Abdul Wali and until now Taliban's leading council have not made any decision on his fate," Taliban spokesman Zabihullah Mujahid told AFP. Police said however they expected the militants to ask for release of their prisoners in Afghan jails in exchange for the release of the senator. The rebels have made similar demands in past abductions. "Taliban have claimed responsibility for the abduction. I think they might ask for the release of some of their prisoners in exchange," said Ghulam Mustafa Muhsini, the deputy police chief of Logar province. The provincial police had on Sunday started an investigation and search for the senator, Mohseni said. Afghan parliamentarians have frequently been targeted in the rising violence in Afghanistan, much of it linked to an insurgency led by the hardline Taliban movement. A roadside bomb blew up a vehicle of a US-based private security firm in southern Afghanistan, killing six Afghan guards, a police chief said Monday. The guards, working for the USPI security firm, were struck Sunday as they travelled through the southern province of Helmand, provincial police chief General Mohammad Hussein Andiwal told AFP. Two other guards were wounded in the attack, near the town of Gereshk. The vehicle that was hit was completely destroyed. There was no immediate claim of responsibility for the attack. The blast was, however, similar to scores of others by the hardline Taliban who are waging a fierce insurgency against the government and its allies.