Four Taliban insurgents were killed and an American soldier injured as a US combat patrol came under fire, a US military official said.

"The US forces were conducting a combat mission in the vicinity of a firebase near Shkin when the engagement occurred," a statement by the US military said. "Two of the soldiers died of wounds received during the initial contact with enemy fighters north-west of Shkin, in Paktika province, this morning."

There are increasing signs that the fundamentalist Taliban regime has been deposed, but not completely defeated, following an upsurge in guerrilla attacks in the past fortnight that have killed more than 100 people, including dozens of government troops.

US fighter jets supporting heavily armed Afghan soldiers have met with stiff resistance in the seven days of fighting in the southern province of Zabul. An American special operations soldier was killed last Friday in a fall during a combat operation there.

"Fighting has escalated today," Khalil Hotak, the US-backed Afghan regime's intelligence chief in the province, told Reuters news agency yesterday. "The bombardment has intensified, so has the shelling, but the Taliban are bitterly resisting."

The clashes in Zabul extend the worst wave of violence in Afghanistan since the Taliban was ousted from power. Much of the bloodshed has been blamed on the militia, which has declared a holy war on foreign troops, aid organisations and their supporters.

By all accounts, the Taliban fighters have suffered the heaviest casualties. A US military spokesman, Colonel Rodney Davis, said on Saturday that 33 Taliban fighters had been killed in the south in recent fighting, but Afghan officials put the insurgent death toll much higher.

The coalition has 11,500 soldiers - 10,000 of them American - hunting down Taliban and al-Qaida fighters, mainly in the south and east of the country. The Dai Chupan district, an area of rugged mountain gorges and ridges, is believed to be a Taliban stronghold from where the guerrillas launch operations in neighbouring provinces.

An Afghan military commander speaking from Larzab, one of the frontlines in the battle, said yesterday that intelligence indicated more Taliban reinforcement fighters had arrived in the area.

"We have information that more than 250 Taliban entered Dai Chupan district from the neighbouring district of Mizan," General Haji Saifullah Khan, who is leading the Afghan soldiers against the Taliban, told the Associated Press.

The offensive against the Taliban in Dai Chupan was launched last Monday, when Afghan officials said US warplanes bombed a mountain hideout near Dozai, killing at least 14 of the insurgents. The fighting has since spilled into other areas in the district. The clashes involve up to 1,000 Afghan troops and a similar number of Taliban guerrillas.

Early yesterday four helicopter gunships and three US fighter jets targeted suspected Taliban hideouts in the Chinaran, Ragh and Kabai areas of Dai Chupan, according to Mr Hotak. Representatives of the Afghan regime say they have the insurgents cornered. The Taliban claim they are holding their own and are digging in for the long fight.

Taliban sources told news agencies that they had shot down a US helicopter last Thursday, killing five American soldiers. Afghan officials deny the claim.

In all, 35 American soldiers have been killed in action in Afghanistan, and 162 injured due to hostilities, the US military said.

· A lack of funds could delay voter registration planned to start in October for next year's Afghan general election, a UN spokesman said yesterday. The UN has said the registration will cost about $80m, but a spokesman, Manoel de Almeida e Silva, said the response had been slow from donor states helping to rebuild the country after two decades of war.

The registration process was due to start in October and last until at least March 2004, before the elections scheduled for June next year.

A commission charged with drafting a new constitution is also experiencing delays, and this could also force the general election to be postponed.