Washington said it would hold Russia accountable for fulfilling the terms of the Ukraine grain deal brokered by Turkey and the United Nations, as it slammed China for stockpiling grain that could be used for those in dire need.

Russia and Ukraine are major global wheat suppliers, but Moscow's Feb. 24 invasion of its neighbor sent food prices soaring, stoking a global food crisis the World Food Programme says has pushed some 47 million people into "acute hunger."

Russia and Ukraine signed a landmark deal in Istanbul on Friday to reopen Ukrainian Black Sea ports for grain exports. The war has stalled Kyiv's exports, leaving dozens of ships stranded and some 20 million tonnes of grain stuck in silos.

U.S. Ambassador to the United Nations, Linda Thomas-Greenfield, said Washington hopes the deal "will help mitigate the crisis Russia has caused," adding that "we will be watching closely to ensure that Russia actually follows through."

The United States also wants to see China help combat the global food crisis, James O'Brien, head of the U.S. State Department's Office of Sanctions Coordination, told reporters.

"We would like to see it act like the great power that it is and provide more grain to the poor people around the world," he said. "China has been a very active buyer of grain and it is stockpiling grain ... at a time when hundreds of millions of people are entering the catastrophic phase of food insecurity," he added.

China's grain stocks at the end of the 2021/22 season were estimated by the International Grains Council to be 323.4 million tons, more than half the global total of 607.4 million. It dwarfs those of the U.S., the world's top grain exporter, which was estimated at 57.8 million tons.

"We would like to see them play more of a role of making the grain available from their own stockpiles and by allowing WFP (World Food Programme) and others to obtain grain," said O'Brien.

He said some 40% of the first grain shipments out of Ukraine in April went to China "which was awkward," adding: "It would have been much better to see that grain going to Egypt, in the Horn of Africa and other places."

China's Embassy in Washington did not immediately respond to a request for comment on O'Brien's remarks.

"We believe it's essential that food, including grain, go everywhere where it's needed," deputy U.N. spokesperson Farhan Haq said on Friday.