TRIPOLI - Rockets rained on several parts of the Libyan capital Saturday despite a truce to end fighting between rival militias, with one projectile hitting a popular hotel and wounding three people, rescuers and medics said.

The renewed violence came as Britain, France, Italy and the United States denounced the violence and warned that an escalation would hamper the political process in North African country.

Rescuers and witnesses said three people were wounded when a rocket hit the Al-Waddan Hotel, which overlooks the bay of Tripoli and is popular with foreigners.

It was not immediately clear if they were clients of staff members.

Rockets also hit other areas of the city, which has been at the epicentre of a bitter power struggle since the fall of dictator Moamer Kadhafi in a 2011 NATO-backed uprising.

On Friday, at least 15 rockets rained on Tripoli and its immediate surroundings, including the capital's only operational airport, according to rescue workers and an airport source.

Since then, authorities have suspended all flights in and out of Mitiga airport, just east of Tripoli, and rerouted them to the airport in Misrata, some 200 kilometres (125 miles) further east.

The fighting broke out on Monday in suburbs south of Tripoli and continued into Wednesday evening after a truce collapsed, despite the United Nations appealing for calm.

The clashes had paused on Thursday after a ceasefire agreement announced by officials from western Libya, but by late afternoon the hostilities had resumed.

In a joint statement, Britain, France, Italy and the United States said they "warn those who tamper with security in Tripoli or elsewhere in Libya that they will be held accountable for any such actions."

They also called on all sides "to refrain from any action that would jeopardise the political framework established by the UN-led mediation to which the international community is fully committed". The interior ministry of Libya's UN-backed unity government, the Government of National Accord (GNA), also denounced the renewed violence Saturday. In a statement, it blamed unnamed factions for "undermining the ceasefire (announced on Thursday)... by blindly launching rockets and shells on Tripoli and its suburbs".

According to the GNA's health ministry, around 40 people have been killed and more than 100 wounded since the fighting first broke out on Monday in the capital's southern suburbs.

Human Rights Watch said four children were among at least 18 civilians killed in the fighting. "The recklessness of armed groups currently fighting each other for power appears to have no boundaries, and civilians are paying the price," said the rights group's Middle East director Sarah Leah Whitson.

"All sides need to do everything they can to spare civilian lives."