Secretary-general says 4,000 more troops will be needed to safeguard August ballot.
Jowzjan has been largely isolated from the rising Taliban violence which has mostly taken place in the southern provinces.
On Thursday, Afghan troops, backed by international forces, said they had killed dozens of anti-government fighters in a gun battle in the southern Helmand province.
“This engagement was yet another blow to the militants, who are quickly losing their ability to operate in Helmand province”
US military statement
The clashes took place after a patrol of Afghan soldiers searching for roadside bombs was targeted by small-arms fire and rocket-propelled grenades in the district of Gereshk, an army spokesman said.
The patrol returned fire, killing 30 fighters, US and Afghan sources said.
“This engagement was yet another blow to the militants, who are quickly losing their ability to operate in Helmand province,” a US military statement said.
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 Helmand 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 the province’s intelligence chief.