Forces loyal to Libyan strongman Khalifa Haftar were pushed back Friday from a key checkpoint less than 30 kilometres (18 miles) from Tripoli, checking their lightning advance on the capital, a security source said. Militiamen from the coastal town of Zawiya, west of Tripoli, retook the base after a "short exchange of fire", the source said on condition of anonymity. The advance by Haftar's forces on Thursday, which came as the United Nations prepares to convene a conference later this month on holding delayed elections, prompted visiting UN chief Antonio Guterres to voice his "deep concern".