Brian Hardesty said the spacecraft intentionally maneuvered dangerously close to a US-registered commercial satellite last weekend, about 340 miles (550 kilometers) above Earth, in what he called deliberately reckless behavior that violated long-established safety thresholds in low-Earth orbit.
The target was ICEYE-X36, a radar surveillance satellite owned by the US subsidiary of Finnish company ICEYE.
Konstantin Vorontsov, deputy head of Russia's UN delegation, dismissed the concerns as "unfounded" and said all Russian space activity is "purely peaceful."
Orbital intelligence firm LeoLabs said Kosmos 2612 held position in "extreme proximity" to the satellite, and a LeoLabs executive told Ars Technica it came within 500 meters and likely much closer.
A US diplomat, Jessup Taylor, suggested the gap may have shrunk to about 100 feet.
An image from Vantor showed the two satellites roughly 1 kilometer apart on October 17.
The approach follows several Russian tests in recent years of suspected anti-satellite weapons, including a system called Nivelir that can fire a projectile at another object in orbit.
Kosmos 2612 backed off without firing and is now far away, though its trajectory does not rule out another approach. The UN debate centered on Russian and Chinese proposals for a treaty banning any weapons in space. US officials call such a ban unrealistic and say the leading space powers already have such weapons; the Pentagon acknowledged in September that the US military has space-based weapons. A further difficulty is defining a space weapon, since ramming and jamming can be done with conventional technology on many kinds of satellites. The US instead points to the 1967 Outer Space Treaty, which bans weapons of mass destruction in orbit.