In a statement, the UN Assistance Mission in Afghanistan (Unama) said that airstrikes on Thursday and Friday killed at least 18 civilians.
It said the Taliban was also to blame, killing at least seven civilians in a suicide bombing on Saturday.
The US military says that it is investigating the air strike incidents.
The pilots have been defended by the Afghan government who pointed out that such events are inevitable when the Taliban use civilians as human shields.
The UN statement said the air strikes were carried out by "international military forces".
But US officials say only American planes have recently been in action over the skies of Afghanistan.
"The people of Helmand have suffered greatly due to the armed conflict in Afghanistan, with 891 civilians killed or injured during 2016," the statement said.
"This figure was the highest in the country in 2016 outside of Kabul."
The statement urged all parties to the conflict "strictly to adhere to their obligations under international humanitarian law" to protect civilians from suffering.
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US military officials say aircraft have carried out about 30 attacks in Helmand over the last week.
"We are investigating the allegations and [are] working diligently to determine whether civilians were killed or injured as a result of US air strikes," Brigadier General Charles Cleveland told the Reuters news agency.
But the families of those killed last week are outraged.
"How could women and children be Taliban?" one man - who said 11 people were killed in his brother's house in Sangin by an air strike on Thursday - asked a correspondent for Reuters news agency.
British forces were stationed in Helmand province since 2006, but left in 2014 when Nato ended its combat mission in Afghanistan.
Since then the insurgency has continued unabated, and the drugs trade fuelling it is still booming.

The pensioner was crossing Argyle Street at its junction with Jamaica Street when he was struck by a bus at about 19:10 on Saturday.
He was taken by ambulance to Glasgow Royal Infirmary where he is being treated for a head injury.
Police appealed for witnesses to the crash to come forward.

The suspect, 28, handed himself in at an east London Police station on Friday, the Met said.
He arrested on suspicion of violent disorder and was bailed until mid-August.
Images of three men still wanted over Tuesday's attack were released by the force.
Bottles and other objects were thrown at the coach as it got stuck in traffic en route to the stadium.
The disorder left four policemen injured and West Ham said it would ban for life any fan found responsible.
Two men, aged 18 and 47, who were arrested for pitch incursion have been bailed to return on a date in late May.
A 20-year-old man who was arrested for throwing bottles at police officers has been bailed to return on a date in August.
The United Nations has said that it is increasingly worried about the high number of civilian deaths in the Afghan province of Helmand.
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An 80-year-old man who was hit by a bus in the centre of Glasgow is in a critical condition in hospital, police have said.
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