The forces of Libya's transitional government have fought their way into the centre of Sirte, one of the last cities loyal to ex-leader Col Gaddafi. Columns of smoke rose above the city as government forces fought their way in, street by street, until they reached the Ouagadougou conference centre where pro-Gaddafi loyalists are holed up. At least 12 people were killed and more than 190 injured, doctors said. Thousands of civilians have left Sirte but many have remained behind. The two sides battered each other with mortar shells, rockets and tank fire in what transitional government forces have described as the final assault on Sirte, some 360km (225 miles) east of the capital, Tripoli. The battle for Sirte comes nearly two months after the former rebels seized control of virtually all of Libya, ousting former leader Col Muammar Gaddafi. The BBC's Jonathan Head in Sirte says Libya has been unable to think about the future until the city falls. Two-pronged assault Troops loyal to the National Transitional Council (NTC) converged on Sirte from Misrata in the west and Benghazi in the east. They faced heavy resistance from pro-Gaddafi snipers and forces as they advanced towards the city centre. The Ouagadougou conference centre, where many of the Gaddafi loyalists are believed to have barricaded themselves, has become the focus of the fighting. There was sustained fire from mortars, machine guns and snipers around the centre, holding off NTC forces. "We entered the Ouagadougou centre compound but fell back because of RPG [rocket-propelled grenade] and sniper fire. There's no cover there," NTC fighter Faisal Asker told the AFP news agency. The UN envoy to Libya, Ian Martin, called on both sides to respect human rights and appealed to the NTC to avoid reprisals. Those accused of war crimes should be detained and brought to justice, he said. "This will lay the foundation for national reconciliation and the future unity of the people of Libya," he added. Checkpoint queues Ambulances brought a steady stream of the wounded to a field hospital west of Sirte. "We are receiving many gunshot wounds, mostly to the head, neck and chest from sniper fire," Dr Ahmed Mohammed Tantoun told the Associated Press. Cars carrying fleeing civilians queued at checkpoints on the roads out of Sirte. The NTC gave civilians the opportunity to leave before the assault began. However, thousands remained in the city, unable to get out or fearful after warnings from pro-Gaddafi fighters that they would be attacked by the interim forces if they surrendered. The NTC's Information Minister, Mahmoud Shamman, told the BBC that civilians in the city were being "kept hostage by Gaddafi fighters". Efforts to negotiate with loyalist commanders have also failed. On Thursday, Col Gaddafi urged Libyans to take to the streets "in their millions" to resist the interim leaders. In a poor-quality audio message broadcast on Syrian-based Arrai television, he said conditions in Libya had become "unbearable", telling people to make their voices heard against "Nato's collaborators" in the NTC. The colonel fled Tripoli when the capital fell to NTC forces in August. His whereabouts remain unknown. Several of his family member are in hiding or have fled the country. Pro-Gaddafi forces also control the desert enclave of Bani Walid, but it is seen as less significant as it does not lead to any exit routes from the country.