Iranian Supreme Leader Ayatollah Ali Khamenei has said he supports moderate President Hassan Rouhani's diplomatic opening to the US at the UN General Assembly last week.
However, he added that some aspects of it were "not proper".
Mr Khamenei did not elaborate on his objections but also said he did not trust the US as a negotiating partner.
He also hinted at disapproval over an historic phone conversation between Mr Rouhani and US President Barack Obama.
Mr Khamenei's overall endorsement is expected to protect Mr Rouhani against conservative hardliners.
Hardliners are opposed to his pursuit of "constructive interaction" with the world to ease Iran's economically crippling isolation.
Mr Khamenei is the ultimate arbiter of high state policy under Iran's unwieldy dual system of clerical and republican rule.
He said prior to Mr Rouhani's trip that he supported "heroic flexibility" in diplomacy.
He cautioned that the Islamic Republic must always remember who its foes are.
The Rouhani-Obama phone call, the first between presidents of the two deeply estranged countries since Iran's 1979 Islamic Revolution, capped a week of overtures by Mr Rouhani and his foreign minister, Mohammad Javad Zarif, to the West.
The landslide election in June of Mr Rouhani has raised hopes of a negotiated settlement to Iran's long-running dispute with world powers over its nuclear programme.
Mr Khamenei will make the final decision on the contours of any deal.
"We support the government's diplomatic movement, including the trip to New York, because we trust the government and we are optimistic regarding it," Mr Khamenei said in a speech quoted by ISNA news agency.
"But some of what happened in New York was not proper, because the US government is not trustworthy, is self-important and illogical, and breaks promises," he said.
Mr Rouhani also won a resounding endorsement for his conciliatory moves at the UN General Assembly from the Iranian parliament.
It is a significant gesture because the hard-line assembly is dominated by factions loyal to Mr Khamenei.