American warplanes were backing Afghan forces against a major Taliban offensive in the south of the country even as the US military pressed on with a troop withdrawal, officials said Wednesday, but insurgents still captured a northern district.

Fierce fighting has erupted in southern Helmand province since the weekend, when the US military formally began withdrawing its remaining troops from Afghanistan.

They were supposed to have pulled out by May 1 under a deal struck with the Taliban last year, but Washington pushed back the date to September 11 -- a move that angered the insurgents.

"The heavy US air strikes against the Taliban positions stopped them from advancing towards Lashkar Gah," said Atiqullah, a local government official, referring to the provincial capital of Helmand.

"The bombing was intense. I have never seen such bombardment in several years."

Attaullah Afghan, head of the Helmand provincial council, told AFP Taliban forces had made advances, but government forces had "retaken some of these areas".

"The Taliban have intensified their attacks in almost all districts of Helmand for four days," he said.

A US defence official confirmed air support was backing government forces.

The US military continues "to deliver precision air strikes in support" of Afghan forces in Helmand and other regions of the country, the official told AFP.

Still, police said insurgents captured the district of Burka in the northern province of Baghlan.

Government forces were in a "tactical retreat", Baghlan police spokesman Jawed Bashrat told AFP, but planned to launch an offensive to retake it.

In the south, thousands fled their homes for refuge in Lashkar Gah in the face of the new fighting.

"The Taliban knocked on our doors and told us to leave. They didn't even give us time to break our (Ramadan) fast," said Gulab Shah.

"This war has turned us into nomads, always on the move."

Haji Shirin said there had been street fighting between the Taliban and government forces.

"We had to spend the entire night in the basement. In the morning I took my family to (Lashkar Gah) city. We couldn't even take our clothes."

A nurse working for Doctors Without Borders described a hail of gunfire.

"There was a lot of shooting, bullets coming into our home. People were afraid, running without shoes, without hijabs, without anything," the nurse said in a tweet posted by the organisation, better known by its French acronym MSF.

It said it had so far treated more than 50 people at its hospital in Lashkar Gah.

The humanitarian organisation Emergency said Tuesday it had received 106 "war wounded" at its facility.

On Wednesday an AFP photographer saw people arriving in Lashkar Gah with their belongings piled into trucks and trishaws.

They were stopped and frisked at the city's entry gates by government forces and directed towards tents set up for the displaced.

Afghan government officials said dozens of Taliban fighters were killed in fighting as they attempted to overrun checkpoints on the outskirts of Lashkar Gah.

The Taliban, meanwhile, said scores of Afghan security personnel had died.

Both sides frequently exaggerate casualties inflicted on the other.

Under the terms of last year's withdrawal deal, the Taliban agreed to stop attacking US troops, but insurgents have continued relentless assaults against Afghan forces.

The missed deadline has raised fears that US troops could again be targeted by the Taliban.

"We have the military means and capability to fully protect our force during retrograde as well as support the Afghan security forces," US top commander in Afghanistan General Scott Miller said ahead of the weekend.

The Afghan defence ministry said late on Wednesday that it had taken control of Camp Mike Span from German forces in the province of Balkh, just days after the US military handed over Camp Antonik in Helmand soon after it began the withdrawal.

The pull out of foreign forces comes even as efforts to hammer out a peace deal between the Afghan government and the Taliban have stalled.