Russian diplomat Alexander Trofimov told the UN Moscow would only use nuclear weapons in response to weapons of mass destruction or a conventional weapons attack that threatened the existence of the Russian state.
“Neither of these two hypothetical scenarios is relevant to the situation in Ukraine,” said Trofimov, a senior diplomat in the non-proliferation and arms control department of Russia's foreign ministry.
Meanwhile, a July 22 UN-brokered deal to unblock Ukrainian grain exports had initial success as the first loaded ship since Russia's invasion anchored safely off the Turkish coast.
The vessel, the Sierra Leone-flagged Razoni, was at the entrance of the Bosphorus Strait, which connects the Black Sea to world markets, around 36 hours after leaving the Ukrainian port of Odesa.
A delegation from the joint co-ordination centre in Istanbul, where Russian, Ukrainian, Turkish and UN personnel work, is expected to inspect the ship early on Wednesday, Turkey's defence ministry said.
It was loaded with 26,527 tonnes of corn.
UN spokesperson Stephane Dujarric told reporters in New York there were about 27 ships in the three Ukrainian ports covered by the export deal that were ready to go.
The exports from one of the world's top grain producers are intended to help ease a global food crisis.
Known as Europe's breadbasket, Ukraine hopes to export 20-million tonnes of grain held in silos and 40-million tonnes from the harvest under way, initially from Odesa and nearby Pivdennyi and Chornomorsk.
Russia has called the Razoni's departure “very positive” news. It has denied responsibility for the food crisis, saying Western sanctions have slowed its exports.