Retaliatory air raids
Hamidullah Zhwak, a spokesman for the local administration, said that the US casualties occurred when a fuel tanker exploded.
A suicide bomber attempted to drive the vehicle towards the base. He was stopped and shot dead but the tanker detonated in the process, Zhwak said.
He said the troops called in helicopters after being attacked and the retaliatory air raids killed at least 32 Taliban fighters.
"After the blast, the American helicopters came in and attacked the Taliban who were preparing to attack the district headquarters," he said.
Zabiullah Mujaheed, a Taliban spokesman, claimed responsibility for Saturday's suicide blast.
"Today Hafiz Omar carried out a suicide attack with 8,000 kilos of explosives in a tanker on an American base," he told the AFP news agency.
"At the same time 100 mujahidin [fighters] attacked the base just after the explosion ... Americans suffered heavy casualties and we lost five of our fighters."
The attack came in the same area where a US soldier, who has been missing since Tuesday, is believed to have been captured by pro-Taliban fighters.
Elsewhere in Afghanistan, seven policemen were killed and two others injured after their vehicle hit a roadside bomb in Registan district in southern Kandahar, the interior ministry told Al Jazeera.
Officials blamed "enemies of the country" for the blast.
The massive military offensive by US marines in southern Afghanistan is the first big push to drive the Taliban out of Helmand province since Barack Obama became the US president in January.
Al Jazeera's Zeina Khodr, reporting from Lashkar Gah, said that the provincial governor had confirmed on Saturday that the US marines and Afghan forces were expanding their area of operation.
"They are now present in Baramshar, which is on the border with Pakistan, a really strategic area where the Afghan president himself has repeatedly requested British troops to take control," she said.
|US troops in Helmand have met local leaders
in an attempt to consolidate gains [AFP]
"Officials here believe that it is where weapons and fighters are smuggled into Afghanistan."
The previous day, officials had reported that the US and Afghan forces had already taken control of the main districts of Nawa and Garamsir, as well as Khananshid.
The Taliban has vowed that thousands of its fighters in the area will fight back, but a military spokesman said that there had been only limited resistance so far.
On Saturday, one security guard died and four others were wounded when a suicide bomber detonated an explosives-packed vehicle near a private security company's four-vehicle convoy in Gereshk district, Daud Ahmadi, a spokesman for the provincial administration, said.
Up to 4,000 marines, backed by Nato aircraft and a 650-strong Afghan force, are moving into towns across Helmand.
Pentagon officials have said that the plan - the largest offensive by the US marines since Vietnam - is not just to inflict casualties against the enemy, but to dig in and hold on to territory.
General David Petraeus, the US chief military commander, has warned that the battle for control over the province will be a "tough fight".
Helmand is one of the Taliban's main heartlands in southern Afghanistan and produces the largest share of the country's opium crop, which supplies about 90 per cent of the world's heroin.