KABUL (Agencies) - Gunmen killed nine Afghan workers who were guarding a NATO supply convoy in the south of the country, police said Monday. The attack in Gereshk district happened on Sunday night, said the deputy police chief for Helmand province, Kamaluddin Khan. Military supply convoys are regularly attacked in Afghanistan, where they are seen as an easier target for insurgents than NATO bases. Separately, airstrikes by international coalition forces killed up to 14 suspected insurgents. An airstrike in northern Baghlan province reportedly killed around 10 people Sunday, a NATO statement said. It had targeted a leader accused of planting bombs and supplying cash and weapons to Taliban leadership in the area. However, NATO was not able to confirm the number of casualties because neither international or Afghan soldiers could get to the area. Another airstrike in southern Nimroz province killed four fighters who were planting a bomb, said provincial police chief Abdul Jabar Pardeli. A Taliban commander was among those killed by Sunday nights strike, he said. In western Afghanistan, a roadside bomb killed three civilians Monday morning in Herat city, police said. Police spokesman Noor Nikzad said the bomb was hidden in a sack in an irrigation ditch. When officers went to inspect the object, it exploded. The three victims had been walking nearby. Nikzad said one police officer was wounded. Meanwhile, NATO forces said that an Afghan detainee captured in an operation in the south was found dead in his holding cell Sunday. The detainee was captured during an operation Saturday and was being held in Kandahar province, NATOs International Security Assistance Force (ISAF) said in a brief statement, adding that the circumstances were being investigated. The NATO and Afghan authorities said they were investigating the death in custody of the man a day after he was detained during a military operation in southern Afghanistan. Afghan authorities were also investigating, an official in the restive southern province said. We learnt this morning from the coalition forces that one of our Afghans has died in their prison, Zalmai Ayoubi, spokesman for the Kandahar governor, told AFP. The name and other details of this individual are not known at this time. Were investigating this. He died last evening. The mans death comes days after a US think-tank said the US military was mistreating detainees at a secret prison in Afghanistan, an allegation rejected by the Pentagon, which said it always treated prisoners humanely. The 16-page report by the Open Society Foundation said Afghans called the secret site Tor Jail or Black Jail and that consistent accounts from detainees referred to inadequate shelter, food or other basic rights. The report said the facility was on Bagram Air Base, outside Kabul, one of the main US military bases in the country. It said some detainees had reported being held there as recently as this year.