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

They were supposed to have been 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 has 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.

"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.

- 'War turned us into nomads ' -

Thousands of Afghans have fled their homes and taken refuge in Lashkar Gah in the face of the new fighting, officials and residents say.

"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, who fled his home on the outskirts of the city earlier this week.

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

Haji Shirin said fighting had erupted on the streets 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 said the fighting was intense.

"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.

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

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

Both sides frequently exaggerate casualties inflicted on the other.

Fighting was also reported in other provinces.

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