It was recently reported in the New York Times that the US government is refusing to issue a visa to Hamid Aboutalebi, who was recently nominated as Iran's ambassador to the United Nations. Since the UN headquarters is in New York, this presumably would prevent Aboutalebi from carrying out his duties.
The article says:
Under a 1947 law that established the headquarters of the United Nations in New York, the United States is obligated to issue visas to diplomats assigned there, even those it finds objectionable. But the United States has reserved the right to turn down people based on concerns over security, terrorism or foreign policy issues.
It's unclear from this in what sense the US is "obligated" to issue visas if they also "reserve the right" not to do so.
Have there been previous incidents where the US has denied a visa to a UN-assigned diplomat? What was the eventual outcome? Does this happen often?
Update: The New York Times has published a followup article. Iran is calling the US's action "unprecedented". The article also claims:
In most previous instances when the United States objected to the entry of a diplomat, the application was quietly withdrawn. But the United States is not known to have ever denied a visa to an ambassador before.
That would answer my question if it's accurate.