Russia on Wednesday rejoined a deal to allow Ukrainian grain exports through the Black Sea after pulling out of the agreement on Saturday.
The revival of the arrangement could ease fears of global food insecurity, but Russian President Vladimir Putin warned Moscow could again pull out of the deal.
The decision by Russia came as Washington said it was "increasingly concerned" that Mr Putin could use nuclear weapons in its campaign in Ukraine.
Moscow had said on Saturday that it was temporarily pulling out of the grain deal, accusing Ukraine of using a safe shipping corridor established under the agreement to launch a drone assault on its Black Sea fleet.
Russia's Defence Ministry said it had now received sufficient guarantees from Kyiv that it would not use the maritime corridor to carry out attacks.
UN Secretary General Antonio Guterres welcomed Russia's decision to rejoin the agreement, which was brokered by the UN and Turkey in July and allows for joint inspections of ships.
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Ukrainian President Volodymyr Zelenskyy said on Twitter that he had thanked Turkish President Recep Tayyip Erdogan for his role in preserving the grain deal.
Mr Zelenskyy hailed its resumption as "a significant diplomatic result for our country and the whole world".
But Mr Putin said Russia could leave the deal again if Ukraine breached its guarantees, although Moscow would not interfere with any grain deliveries even if it did.
The Kremlin had warned that the route was dangerous for shipments without its participation in the agreement but some deliveries from Ukraine still went ahead on Monday and Tuesday.
A Turkish security source said the corridor was open again from 9am GMT although no departures from Ukraine were planned on Wednesday.
Russia's Deputy Foreign Minister Andrei Rudenko said Moscow had yet to decide if it would remain part of the deal after November 18.
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The agreement comes up for renewal on November 19, but the extension is a separate issue and that decision will be made "taking into account all the accompanying factors", state news agency RIA Novosti reported Mr Rudenko as saying.
US State Department spokesman Ned Price on Wednesday called for the deal to be renewed, saying this "will ultimately inject even more predictability and stability in this marketplace and, most importantly, apply downward pressure to the prices" of global food.
The deal, overseen by the Joint Co-ordination Centre in Istanbul, has allowed more than 9.7 million tonnes of grain and other foodstuffs to leave Ukrainian ports.
This has brought much-needed relief to a global food crisis caused largely by the conflict between Russia and Ukraine, both major global grain exporters.
World grain prices, which had soared this week, began to ease on Wednesday after Russia announced it was returning to the deal, despite doubts over its future.
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Mr Putin had demanded "real guarantees", while Mr Zelenskyy on Tuesday urged "reliable and long-term protection" of the corridor.
The Russian Defence Ministry said it obtained written guarantees from Kyiv.
It said Ukraine guaranteed "the non-use of the humanitarian corridor and Ukrainian ports determined in the interests of the export of agricultural products for conducting military operations against the Russian Federation".
The White House said repeated discussion by Russian officials of the potential use of nuclear weapons in Ukraine has left US officials worried that a risk could become a reality.
"We have grown increasingly concerned about the potential as these months have gone on," said White House national security spokesman John Kirby.
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Mr Kirby also said North Korea was sending a significant amount of artillery ammunition to Russia under cover of shipments to countries in the Middle East or North Africa.
He did not confirm a report in The New York Times that high-level Russian military officials recently discussed when and how they might use tactical nuclear weapons on the battlefield.
The report, which quoted unidentified US officials, said Mr Putin did not take part in the discussions and there was no indication that the Russian military had decided to use the weapons.
Russia's Foreign Ministry said the world's "top priority" should be to avoid a clash of nuclear powers.
"We are firmly convinced that in the current difficult and turbulent situation — a consequence of irresponsible and shameless actions aimed at undermining our national security — the top priority is to prevent any military clash of nuclear powers," the ministry said.
