Explosions shook the Libyan capital Tripoli late Saturday after an airstrike, residents said, in an escalation of a two-week offensive by eastern forces on the city held by the internationally recognized government.

A Reuters reporter and several residents said they saw an aircraft circling for more than 10 minutes over the capital with a humming sound before opening fire on a southern suburb, scene of the heaviest fighting between the rival forces.

Reuters was unable to confirm whether an aircraft or unmanned drone was behind the strike which triggered heavy anti-aircraft fire. Residents had reported drone strikes in the past days, but there has been no confirmation and explosions heard in the city centre this time were louder than in previous days.

Residents counted several missile strikes which apparently hit a military camp of forces loyal to Tripoli in the Sabaa district. Authorities had closed Tripoli's only functioning airport but reopened it hours later, the airport's website said.

The Libyan National Army (LNA) force loyal to commander Khalifa Haftar started an offensive two weeks ago but has been unable to breach the government's southern defences.

If a drone strike was confirmed, this would point to more sophisticated warfare. The LNA has so far mainly used aging Soviet-made jets from the air force of Moammar Gadhafi, toppled in 2011, lacking precision firepower and helicopters, according to residents and military sources.

Libya's UN-backed unity government has been getting support from the Misrata militia, seen here near gun-mounted vehicles as they prepare to move to the frontline, closer to the capital, on April 9. (EPA-EFE)

In the past the United Arab Emirates and Egypt have supported Haftar with airstrikes during campaigns to take eastern Libya. Both countries flew airstrikes on Tripoli in 2014 during a different conflict to help a Haftar-allied force, U.S. officials said at the time.

Since 2014, the UAE and Egypt have provided the LNA with military equipment such as aircraft and helicopters, helping Haftar to gain the upper hand in Libya's eight-year conflict, past UN reports have established.

The UAE even built an air base in Al Khadim in eastern Libya, one such report said in 2017.

The airstrikes, which were also filmed by residents in video posted online, came after a day of heavy clashes in southern districts, with shelling audible in the city centre.

U.S. president speaks with eastern commander

The violence spiked after the White House said on Friday that U.S. President Donald Trump spoke by phone with Haftar earlier in the week.

The disclosure of the call and a U.S. statement that it "recognized Field Marshal Haftar's significant role in fighting terrorism and securing Libya's oil resources" has boosted the commander's supporters and enraged his opponents.

Western powers and the Gulf have been divided over a push by Haftar's forces to seize Tripoli, undermining calls by the United Nations for a ceasefire.

Both sides claimed progress in southern Tripoli on Saturday, but no more details were immediately available.

A Reuters TV cameraman visiting the southern Khalat Furgan suburb on Saturday heard heavy shelling but saw no apparent change in the frontline. Shelling continued in the southern outskirts of the capital on Sunday.

On Friday, two children were killed in shelling in southern Tripoli, residents said. The fighting has killed 220 people and wounded 1,066, the World Heath organization (WHO) said.

It was unclear why the White House waited several days to announce Monday's phone call.

On Thursday, both the United States and Russia said they could not support a UN Security Council resolution calling for a ceasefire in Libya at this time.

Russia objects to the British-drafted resolution blaming Haftar for the latest flare-up in violence when his LNA advanced to the outskirts of Tripoli earlier this month, diplomats said.

The United States did not give a reason for its decision not to support the draft resolution, which would also call on countries with influence over the warring parties to ensure compliance and for unconditional humanitarian aid access in Libya.