Lieutenant-Colonel Jerry O'Hara, a US military spokesman, said American forces, including aircraft, responded to an initial attack on Afghan security forces on Friday.
He said there were no reports of casualties among the US troops, and that the fighting was continuing. Sixteen Taliban fighters were said to have been killed.
Helmand has been plagued by insurgents since US forces and their Afghan allies ousted the Taliban in late 2001. It is also a region where poppy growing and drug-smuggling is rife.
Several thousand British troops will be deployed there later this year under a plan to expand Afghanistan's peacekeeping force led by Nato.
After weeks of political wrangling and international pressure, the Dutch parliament also agreed on Friday to send 1400 of its troops to southern Afghanistan.
Mullah Mir, Helmand's deputy governor who was in a police convoy that came under attack, said the violence began when gunmen ambushed a police convoy, killing a policemen.
It was the biggest attack in Afghanistan for months.
Dozens of people, most of them civilians, have been killed in a wave of attacks - including 14 suicide bombings - across the south and east in recent months but there have been no clashes on a large scale.
The US military said earlier that the Taliban were increasingly turning to bomb attacks on military and soft civilian targets after suffering heavy losses in battles with US and Afghan government troops last summer.