The US army said they had captured several suspects in Wednesday's offensive and seized about 30kg of opium.
Casualty figures have not been reported due to security restrictions.
Attacks on security forces have increased as Afghans gear up for a presidential election scheduled for August 20.
However, the attacks have come mainly in the south where Taliban fighters have their strongest presence.
The new offensive, named Eastern Resolve 2, aims to gain control of strategic areas within the southern valley, where the Taliban are solidly entrenched.
By occupying Dahaneh, US forces hope to isolate the Taliban in woods and mountains, away from civilian centres.
Captain Zachary Martin, the US army commander leading the assault, said: "I think this has the potential to be a watershed".
He quoted by the Associated Press said the goal is to cut off the Taliban from a major rear base, and reclaim the area's main market district.
It is hoped this would have a ripple effect through neighbouring villages, making civilians more willing to co-operate with Nato forces.
"In the long term, it could have tremendous effects for the entire province," Martin said.
Dahaneh serves as a main trading route through northern parts of Helmand, which produces 60 per cent of the world's opium.
|The Taliban has vowed to disrupt the forthcoming presidential election [AFP]
A combined force of 500 US and Afghan troops took part in Wednesday's attack.
Compound-by-compound searches were also conducted by Afghan forces that accompanied the Americans.
Martin said the US forces had devised tactics to minimise civilian casualties in the densely populated area.
He said troops would strictly limit the type of weapons they used and would stick to a "proportional response" when under fire.
Thousands of Nato troops are conducting offensives in Helmand an area where roadside bombs are frequently used to attack foreign and Afghan forces.
At least 14 Nato troops, including at least seven Americans, have died in bomb blasts this month.