Ukrainian President Volodymyr Zelenskiy on Tuesday said that despite arms supplies from the West, his country's forces could not yet overcome Russian advantages in heavy guns and manpower.

"This is very much felt in combat, especially in the Donbas. ... It is just hell there. Words cannot describe it."

Donbas, Ukraine's traditional industrial heartland in Ukraine's east, has seen some of the war's heaviest fighting.

Russia sent tens of thousands of troops into Ukraine on Feb. 24 in what it calls a "special military operation". Kyiv and the West have condemned it as an unprovoked war of aggression.

At a U.N. conference on Tuesday, Igor Vishnevetsky, deputy director of the department for non-proliferation and arms control of the Russian foreign ministry, refuted all allegations of "unprovoked aggression". He also added that Moscow was convinced a nuclear war "must never be fought".

Russian diplomat Alexander Trofimov told the United Nations that 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.

"None 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.

SAFE PASSAGE

Meanwhile, a July 22 U.N.-brokered deal to unblock Ukrainian grain exports had an initial success as the first loaded ship since Russia's invasion safely anchored 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 1800 GMT on Tuesday, some 36 hours after leaving the Ukrainian port of Odesa.

A delegation from the Joint Coordination Centre (JCC) in Istanbul, where Russian, Ukrainian, Turkish and U.N. personnel work, is expected to inspect the ship at 0700 GMT on Wednesday, Turkey's Defence Ministry said.

It was loaded with 26,527 tonnes of corn.

U.N. spokesman 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 now 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.

Reuters