Catherine Russell said the situation in gang-wracked Haiti was “catastrophic” and growing worse “by the day”.
She told a meeting of the UN Security Council that in many areas essential services had collapsed.
Meanwhile, a transitional presidential council has still not been sworn in.
While Haiti has been blighted by gangs for years, the armed groups stepped up their attacks at the end of February when Prime Minister Ariel Henry travelled to Kenya to seal a deal for a multinational security force to be deployed to the Caribbean nation.
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Mr Henry was prevented from returning to Haiti as the gangs attacked the international airport, forcing its closure.
He agreed last month to step aside as soon as a transitional presidential council was created and remains abroad.
However, the formation of the council has been beset by delays and internal disagreements. The council’s nine members have yet to be sworn in before they can take over power.
Even though the gangs said that their main aim was the ouster of Mr Henry, they have continued to carry out attacks on the police force.
They have also looted universities and libraries, torched pharmacies and forced the closure of the capital Port-au-Prince’s largest hospital.
Non-governmental groups say the armed groups now control an estimated 90% of Port-au-Prince.
According to Unicef, 2.7m people live in areas under effective gang control with children often becoming victims of their attacks.
“Each day, children are being injured or killed. Some are being recruited, or they are joining armed groups out of sheer desperation,” Ms Russel told the UN Security Council meeting in New York on Monday.
“Recent Unicef data indicates that anywhere from 30% to 50% of armed groups in Haiti currently have children within their ranks,” she added.
Local media reported attacks in the poor Lower Delmas neighbourhood of Port-au-Prince over the weekend.
And on Monday, the Varreux terminal – through which most of the fuel destined for the capital passes – was forced to suspend operations as armed men seized trucks and blocked access roads.
Clashes between the police and gang members were also reported in the vicinity of the National Palace, the building where the transitional council is due to be sworn in.
According to local media, at least one person was killed in shooting in the streets adjacent to the palace.
