Haftar's order to the self-styled Libyan National Army came as UN chief Antonio Guterres was in Tripoli.
Armed groups from the western city of Misrata, which back the government, have vowed to stop any advance.
Libya has been riven by violence and division since long-time ruler Muammar Gaddafi was deposed and killed in 2011.
Guterres, the US
Speaking to reporters in Tripoli, Guterres said he was making a "strong appeal to stop... the escalation,” according to the BBC.
The UN Security Council will meet on Friday to discuss the situation following a request from the UK, reports said.
The US, UK, France, Italy and the United Arab Emirates (UAE) issued a joint statement appealing for calm.
"At this sensitive moment in Libya's transition, military posturing and threats of unilateral action only risk propelling Libya back toward chaos," the statement, issued by the US state department, said.
"We strongly believe that there is no military solution to the Libya conflict," the governments added.
The UN had been planning to hold a conference in Libya later this month for talks over ending the country's long-running crisis.
After Gen Haftar's announcement, his forces moved towards the capital from several directions, one of his spokesmen said.
The Libyan National Army (LNA) says it has secured Gharyan and moved on.
However, it said two of its soldiers had been wounded in clashes in a nearby area.
A Gharyan official told AFP that there were "ongoing efforts to avoid a confrontation" between rival fighters in the town.
The UN-backed government in Tripoli said it had put its forces on high alert.
Meanwhile, residents in Misrata said armed groups from the city had begun moving towards the Libyan capital, Reuters reported.