Kai Eide finishes his term in March
The UN representative to Afghanistan says "negative trends" threaten to make the country's situation unmanageable.
Kai Eide, in his final formal address to the UN before his term ends in March, said he was worried about the strength of the Taliban insurgency.
He was also concerned by growing frustration among Afghans over expectations that have not been met.
Mr Eide urged a focus on building up civilian institutions so Afghans could govern themselves.
Although training of police and the army was important, a military strategy was not enough to ensure that Afghans were able to take control, he told the Security Council in New York.
Education, agriculture and infrastructure were also important to boost the country's economy, with key civilian institutions needed to enable the government to operate, he said.
President Hamid Karzai suffered a setback at the weekend when most of his cabinet nominees were rejected by parliament - effectively leaving Afghanistan without a fully functioning government.
Mr Karzai is expected to offer new nominees within days.
Mr Eide told the UN that "negative trends" included growing impatience among the public both inside and outside Afghanistan.
"I am worried about increasing frustration in the Afghan public over what they see as expectations that have not been met.
"And I'm worried about the difficulties of the international and Afghan forces in putting the insurgency on the defensive.
"If these negative trends are not reversed - and reversed soon - then there is a risk that they will... become unmanageable."
President Karzai was returned for a second five-year term after last August's election, despite investigators discovering more than a quarter of votes were fraudulent.
Mr Eide said there was "international interference" before and after the election.
He said Afghans perceived that they were treated in a disrespectful, and sometimes arrogant, manner by countries that saw their nation as a "no man's land", rather than a sovereign state.
He strongly emphasised the need for such treatment to end.
While none of Mr Eide's ideas was especially new, perhaps the most damning aspect of his analysis was that all these issues still need to be addressed, says the BBC's Matthew Price at UN headquarters.