Heavy fighting has erupted in the Libyan city of Zawiya, west of Tripoli, as Col Muammar Gaddafi's forces try to re-establish control over the country. Reports said government forces using artillery and anti-aircraft guns had attempted to take the main square. But a resident told the BBC that tanks had been captured and burnt, government troops had retreated, and the city centre was now back in rebel hands. Heavy casualties have been reported there and in other key cities. Dozens of people were also killed and hurt in apparently accidental blasts at an arms dump in rebel-held Benghazi on Friday. Col Gaddafi has also launched a counter-attack on the diplomatic front, appointing a new ambassador to the United Nations to replace the envoys who have come out against him. But it is not clear whether Washington will grant the new envoy, Ali Abdussalam Treki, a visa to allow him to travel to New York. Conflicting reports. The fighting in both Zawiya and Ras Lanuf, an oil port east of Sirte, has been marked by conflicting claims. Reports from Zawiya - just 50km (30 miles) west of Tripoli - said the town had come under attack from both east and west from well-armed government forces. At least 18 people were killed in the fighting. In an interview with the BBC World Service's World Today programme, Zawiya resident Mohammed said the fighting had began at about 0600 local time (0400 GMT) with government forces attempting to retake the city. But he said that by 1100 rebel forces had beaten them back. Pro-Gaddafi troops "came from east and west and they took up positions in high-rise buildings... and started shooting", Mohammed said. "Some tanks went to the square and were captured and burnt. There were some casualties among Gaddafi's troops and our troops - but Zawiya was never captured by Gaddafi's troops, this is confirmed. "I am outside the square, Gaddafi's troops are nowhere to be seen, they have all fled. Some lost their vehicles so they had to walk, from one street to another hiding in buildings. But the rebels got them, some were killed, some captured." He said it was likely that pro-Gaddafi troops had retreated to the outskirts of the city and said it was possible they could launch another assault on the city. Fierce fighting was also reported in and around Ras Lanuf, with the sound of multiple explosions and heavy artillery being heard after opposition fighters advanced on the city. There were other reports of a counter-attack by pro-Gaddafi forces, which withdrew to Ras Lanuf two days ago after a battle a little further east in Brega. Reports said tanks had fired at residential houses. Source: BBC