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.

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 now says they will only be out by September 11 -- a slippage 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."

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

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.

Dozens of Taliban fighters were killed in fighting on the outskirts of Lashkar Gah as insurgents attempted to overrun several checkpoints, Afghan government officials said.

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 not to attack US troops -- but insurgents have continued relentless assaults against Afghan forces.

The missed deadline raised fears, however, that the Taliban would resume targetting foreign forces even as they completed their withdrawal, or that Afghan troops would be left defenceless without air support.

"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 Afghan government and the Taliban have stalled.