By Pavel Polityuk

KYIV The U.N. has proposed that Kyiv, Moscow and Ankara start preparatory work for the transit of Russian ammonia through Ukraine as it tries to salvage a deal allowing safe Black Sea grain exports, a source close to the talks said on Wednesday.

As the preparatory work starts, the U.N. wants parallel talks to be held on widening the Black Sea deal that was agreed last July to include more Ukrainian ports and other cargoes, said the source, who spoke to Reuters on condition of anonymity.

Russia agreed this month to a two-month extension of the deal but has said the initiative will cease unless an agreement aimed at overcoming obstacles to Russian grain and fertiliser exports is fulfilled.

Ukraine and Turkey have agreed to the new proposal, intended to improve operations in the Black Sea grain export corridor, but Russia has not yet responded, the source said.

The United Nations had no immediate comment.

The U.N. and Turkey brokered the Black Sea Grain Initiative between Moscow and Kyiv last July to help tackle a global food crisis aggravated by Russia's invasion of Ukraine, a leading global grain exporter.

The U.N. has handed "an official appeal to the leaders of Ukraine, Turkey and Russia with a proposal for a specific mutually beneficial algorithm to radically improve" the work of the grain corridor, the source told Reuters.

"Ukraine and Turkey have confirmed their readiness to work on the algorithm proposed by the Secretary General. At the same time, as of May 30, Russia has not given its consent, despite the presence of favourable positions in the algorithm."

Ukrainian officials have said that since mid-April, Russia has "unreasonably restricted" the work of the Black Sea grain deal.

Russia has denied this and urged all parties to unblock the transit of ammonia via the Ukrainian port of Pivdennyi, near the Black Sea port of Odesa, which was halted after Russia's invasion in February last year.

Ukrainian authorities have said workers would need about 30 days to prepare the pipeline to pump ammonia again.

Ukraine's deputy renovation minister said on Tuesday that Kyiv was seeking guarantees from Moscow and the U.N. that the grain deal will work normally if Ukraine allows Russia to export ammonia via the pipeline.

A senior government source told Reuters this month that Kyiv would consider allowing Russian ammonia to transit its territory for export on condition that the Black Sea grain deal is expanded to include more Ukrainian ports and a wider range of commodities.

(Reporting by Pavel Polityuk, Editing by Timothy Heritage and Kirsten Donovan)