An attack on a checkpoint between the Libyan capital and the town of Zliten killed six soldiers of the UN-backed unity government on Thursday, an interior ministry source said."The attack was carried out with grenades and light weapons at 07:45," Mayor Moftah Ahmadi earlier told AFP, giving a lower death toll of four.But an interior ministry source, asking not to be named, said two others were seriously wounded and had also died, raising the number killed to six, all of them soldiers.A building beside the checkpoint was left riddled with bullet holes and streaked with trails of blood on the ground, while the mangled remains of a car stood outside.According to Libyan media, most of the soldiers were killed inside the building used as a rest quarters for the troops at the checkpoint in Zliten, which lies 170km from the capital in an area of western Libya under the control of the beleaguered Tripoli-based government.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookThe mayor said the checkpoint was manned by special forces from the interior ministry, on the main highway along the Mediterranean coast."According to preliminary reports, there were three attackers and one of them was killed in the exchange of fire," Ahmadi said, adding that the two others had escaped."A security perimeter has been set up in the zone around the town," the ministry source said, to hunt down the perpetrators.Zliten security chief Mohammed Abu Hajar told Libya's Al-Nabaa television that the attack was the work of the Islamic State group, although there was no immediate claim of responsibility from the jihadists.ISIS took advantage of the anarchy that reigned in Libya after the NATO-backed overthrow and killing of longtime dictator Muammar Gaddafi in 2011 to establish footholds in several parts of the country.In June 2015, they seized control of Gaddafi's hometown of Sirte, only ceding it in December of the following year in the face of an offensive by the Tripoli-based government's forces and allied militias.The jihadists have continued the fight from rural areas of western Libya, including around Zliten, and in April the Tripoli-based Government of National Accord launched a campaign to flush them out.The GNA has struggled to assert its authority outside western Libya since its formation in 2015. The east, including second city Benghazi, is largely in the hands of the self-styled Libyan National Army of military strongman Field Marshal Khalifa Haftar.