TRIPOLI (Reuters) - Heavy clashes erupted on Sunday between rival factions on the coastal road east of the Libyan capital Tripoli, according to a witness and local reports.

The clashes broke out when an armed group opposed to the U.N.-backed Government of National Accord (GNA) in Tripoli tried to approach the capital and were confronted by forces that have aligned themselves with the GNA, the witness said.

The fighting exposes the GNA’s vulnerability to militias that gained influence during and after Libya’s 2011 uprising.

The anti-GNA forces, which are aligned with a previous, self-declared government, were driven from Tripoli late in May and have been trying to regroup. The GNA recently issued a warning about a counter-attack on the capital.

Since the GNA arrived in Tripoli last year it has co-opted some of the many armed groups that had a presence in the capital. But it has made little progress integrating them and the security situation has remained highly volatile.

Groups aligned with the self-declared national salvation government and largely drawing on support from the western city of Misrata have battled to hold their ground.

The two sides clashed on Sunday near the coastal town of Garabulli, about 50 km (30 miles) east of Tripoli.

“The ground is shaking under my feet,” said a resident who asked not to be named. “The noise from the clashes is so loud that I think the groups fighting are using heavy artillery.”

GNA-aligned brigades closed the coastal road in Tajoura, an eastern suburb of Tripoli, and built sand barriers to try to block their rivals’ advance.

Local authorities were advising residents to evacuate and stay away from the area. The U.N. mission to Libya urged both sides to refrain from further escalation.