Secretary-general says 4,000 more troops will be needed to safeguard August ballot.
One Afghan soldier was reported wounded in the clash.
“This engagement was yet another blow to the militants, who are quickly losing their ability to operate in Helmand province,” a US military statement.
The number of fighters killed is the biggest from a single clash announced by the military in more than two months.
There was no independent confirmation of the toll and the Taliban has disputed the claims.
Earlier on Thursday, Dad Mohammad Khan, a prominent Afghan politician, was killed by a roadside bomb in the same province.
Khan, a member of the Afghan parliament, was a prominent critic of the Taliban, which lead the government from 1996 until 2001 when it was overthrown by a US-led invasion.
Shir Mohammad Akhonzada, the former governor of Helmand, said that Khan fled the country during Taliban rule but later returned as Helmand’s intelligence chief.
Taliban fighters have maintained their armed struggle to regain power from the Western-backed administration.