KABUL - At least 31 Taliban militants were killed by Afghan security forces backed by US air strikes in Afghanistan's central Ghazni province, as the Afghan army battled to protect a key highway, officials said on Saturday.Mohammad Arif Noori, the spokesman for the provincial governor, said the militants were planning to wrest control of an arterial road and had attacked many security check posts."With the help of US air forces, the Taliban militants have been pushed back from the areas near Ghazni-Paktika highway, but the road is still closed due to serious damages caused by the Taliban," said Noori.During clashes on Friday, two civilians were killed and four were injured when a mortar shell hit a home in the Andar district. Andar is one of the unstable districts of Ghazni province, 95 miles (153 km) southwest of the capital, Kabul.In a separate incident, two militants were killed when explosives went off on a highway in Ghazni.The Taliban confirmed the clashes. The group's spokesman, Zabihullah Mujahid, however, gave a conflicting casualty figure.According to Mujahid, nine Afghan soldiers and one Taliban fighter were killed. He said 10 Afghan soldiers and three Taliban fighters were wounded in the battle to control Andar district.In April, the militants killed a district governor in Ghazni, his bodyguards and five intelligence agents.The Taliban have stepped up attacks across the country, after they announced their annual spring offensive, a start of the fighting season as the weather allows easier maneuverability through Afghanistan's mountains.This week suicide bombers in the capital Kabul killed at least 26 people, including nine journalists.According to the top US watchdog on Afghanistan, the number of Afghan security forces fell by nearly 11 percent in the past year, and the vast majority of Afghan forces are incapable of preventing the Taliban from capturing territory.