Haftar leads a self-styled Libyan National Army (LNA) with a base in the eastern city of Benghazi. LNA is the main opponent of the UN-backed Libyan government based in the capital Tripoli in the west.
On Thursday, forces loyal to Haftar clashed with rival forces south of Tripoli, LNA representatives told Reuters.
United Nations Secretary-General António Guterres, who is on a visit to Libya to bring rival leaders across the country together for a national reconciliation conference, tweeted:
"I am deeply concerned by the military movement taking place in Libya and the risk of confrontation. There is no military solution. Only intra-Libyan dialogue can solve Libyan problems. I call for calm and restraint as I prepare to meet the Libyan leaders in the country."