Russia on Wednesday rejoined a deal to allow Ukrainian grain exports through the Black Sea, but President Vladimir Putin warned Moscow could again pull out of the agreement.
The revival of an arrangement aimed at easing fears of global food insecurity came as Washington said it was "increasingly concerned" Russia 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 resume participation in the agreement, which was brokered by the United Nations and Turkey in July and allows for joint inspections of ships.
President Volodymyr Zelensky said on Twitter that he had thanked Turkish President Recep Tayyip Erdogan for his role in preserving the grain deal. The Ukrainian leader later hailed its resumption as "a significant diplomatic result for our country and the whole world".
But Putin said Russia could leave the deal again if Ukraine violates its guarantees, though Moscow would not interfere with any grain deliveries even if it did.
Moscow had warned the route was dangerous for shipments without its participation in the agreement but some deliveries from Ukraine still went ahead on Monday and Tuesday.
Real guarantees
A Turkish security source said the corridor was open again from 09:00 GMT although no departures from Ukraine were planned Wednesday.
Russia's Deputy Foreign Minister Andrei Rudenko said Moscow has yet to decide if it would remain part of the deal after November 18.
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 him as saying.
US State Department spokesman Ned Price called Wednesday 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 Coordination Centre in Istanbul, has allowed more than 9.7 million metric tonnes of grain and other foodstuffs to leave Ukrainian ports.
This has brought much-needed relief to a global food crisis triggered by the conflict between Russia and Ukraine, both major global grain exporters.
World grain prices, which had soared earlier this week, began to ease on Wednesday after Russia announced it was returning to the deal, despite doubts over its future.
Putin had demanded "real guarantees", while Zelensky on Tuesday had 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".
Turbulent situation
The White House said repeated discussion by Russian officials of the potential use of nuclear weapons in Ukraine has left US officials worried that 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.
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 New York Times report that high-level Russian military officials recently discussed when and how they might use tactical nuclear weapons on the battlefield.
The report, which cited unnamed US officials, said Putin did not take part in the discussions and there was no indication the Russian military had decided to deploy 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," a foreign ministry statement said.
