The United Nations has invited Kiev, Moscow and Ankara to begin preparatory work for the transit of Russian ammonia through Ukraine in an attempt to save an agreement on the safe export of grain through the Black Sea, a source close to the talks said on Wednesday.

As preparations begin, the UN wants to hold parallel talks on expanding the Black Sea Agreement, agreed last July, to include more Ukrainian ports and goods, the source said, speaking to Reuters on condition of anonymity.

This month, Russia agreed to extend the deal for two months, but said the initiative would end if a deal aimed at removing barriers to Russian grain and fertilizer exports was not reached.

• Grain supplies from Ukraine across the Black Sea are blocked, despite the extension of the agreement. There are dozens of ships around Istanbul

• Lavrov warns the West: the Black Sea grain agreement is at risk of collapse

Ukraine and Turkey have agreed to a new proposal aimed at improving operations in the Black Sea grain export corridor, but Russia has yet to respond, the source said.

The United Nations had no comment.

Last July, the UN and Turkey brokered the Black Sea Grain Initiative between Moscow and Kyiv to help deal with a global food crisis exacerbated by Russia’s incursion into Ukraine, a major global grain exporter.

The UN has turned to the leaders of Ukraine, Turkey and Russia with “an official appeal with a proposal for a specific mutually beneficial algorithm for radical improvement” of the work of the grain corridor, a 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 did not agree, despite the presence of favorable positions in the algorithm.”

Ukrainian officials said that since mid-April, Russia has “unreasonably limited” work on the Black Sea grain agreement.

Russia denies this and is calling on all parties to unblock the transit of ammonia through the Ukrainian port of Pivdnyi, near the Odesa port, which was closed after the Russian invasion last February.

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

Ukraine’s deputy reconstruction minister said on Tuesday that Kyiv is demanding guarantees from Moscow and the United Nations that the grain deal will work properly if Ukraine allows Russia to export ammonia through the pipeline.

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