MOSCOW — In Kazakhstan, they are holding on to their flour and wheat. In Vietnam and India, it's rice. In Cambodia, it's fish and rice.

With grain-export giants Russia and Ukraine, the world is watching their next moves.

A steady rise in countries limiting or banning food exports has triggered warnings from U.N. food agency leaders about possible disruptions to the global food supply as the world retrenches amid the coronavirus pandemic — potentially making critical staples such as wheat and rice more costly and harder to find.

For exporting countries, domestic reserves of grains and rice are generally good after years of relatively good harvests, according to the United Nations’ food and agriculture program.

But reductions in exports could hit countries in Africa, the Philippines, the Persian Gulf and other regions that import much of their food.

Prices for rice and wheat are starting to climb as some countries, such as Saudi Arabia and Egypt, go on a buying spree to boost their stockpiles. Other concerns include how long closures of borders and transport networks will disrupt global food supplies.

But protectionist moves by some food-exporting countries — to hold on to their products in a time of crisis — raise wider issues of global food security and the hard-edge competition between nations for medical supplies and other goods even as the world faces a common enemy.

Why the U.N. is worried

“Now is the time to show solidarity, act responsibly,” the U.N.’s Food and Agriculture Organization said last week in a statement.

“We learned from previous crises,” the statement continued, “that such measures are particularly damaging for low-income, food-deficit countries and to the efforts of humanitarian organizations to procure food for those in desperate need.”

But when one country acts, others tend to follow.

On March 25, Vietnam, the world’s third-largest rice exporter, halted new rice export contracts, then revised the decision Tuesday in favor of a quota. Last week, India’s powerful Rice Exporters Association announced it would halt new rice contracts.

Cambodia’s leader, Prime Minister Hun Sen, ordered a ban Tuesday on all rice and fish exports and asked people to plant vegetables and other crops for local markets “during this difficult time.”

In Turkey, lemon exports are being curbed to ensure enough for use in a popular lemon-scented mixture often splashed on hands and face. Serbia has announced a ban on exports of sunflower oil. Egypt has barred exports of legumes for three months.

“If these spread to more major exporters and more widely traded products . . . it could get dire indeed,” said Cullen S. Hendrix, senior fellow at the Peterson Institute for International Economics, based in Washington.

As recently as 2011, food export bans in Ukraine and Russia had a devastating impact, leading to hunger and protests in Africa, and potentially contributing to Arab Spring unrest.

“This is why the major exporters of the [Group of 20 industrialized nations] need to be clear in their commitments to maintaining open markets and trade,” he added, calling export bans “a blunt tool that causes blunt-force trauma.”

Pledges for exports to flow

So far, the West and many allies have made no moves to put up barriers to food exports — even as border closures and travel restrictions keep farmworkers from tending the fields.

President Trump has pushed American agricultural exports even as the United States became the center of the pandemic. Separately, a range of nations, including Australia, Canada, Chile and New Zealand, have pledged to keep trade and food flowing.

David Laborde, senior research fellow at the Washington-based International Food Policy Research Institute (IFPRI), said there is no crisis yet, with global food stocks relatively high. So far less than 1 percent of the global food value, in calorie terms, is affected by the export restrictions.

But he warned that the danger of a handful of nations imposing export quotas or moratoriums has a cascading effect that could clog global markets.

“In the last few days, we have a small country like Moldova which was saying, ‘Oh, because Ukraine and Russia are taking measures, we should do it, even if actually we don’t need it.’ This imitation, this domino effect is something we always have to worry about,” he said.

Opposition to the curbs

The main concerns are Vietnam’s moratorium on new rice export contracts, and Kazakhstan’s move to slash exports of wheat and flour, which will hit neighboring countries in Central Asia.

On March 22, the Kazakhstan government announced a ban on exports of flour and other key food products despite ample reserves.

The country’s farmers and grain millers pushed back, especially after finally breaking into the coveted Chinese market in recent years. The total ban was soon reversed, but the government slapped on restrictive export quotas on flour and wheat.

“The case of Kazakhstan in the flour restriction may have a very strong impact regionally in Central Asia and Afghanistan that get most of their supply of wheat product from Kazakhstan,” Laborde said. “The specific case of Central Asia, is, I would say, of concern.”

When Chinese customers called the director of a top Kazakh flour mill last week, director Dos-Mukasan Taukebayev had to turn them down.

“If the restrictions are not lifted, then our industry will collapse. Many enterprises will go bankrupt, and thousands of people will lose jobs,” said Taukebayev, director of Karaganda-based Mutlu flour mill, one of the nation’s biggest. “Exports are vital to us.”

Russia and Ukraine also have reversed or at least partially walked back grain export restrictions, although Russia said it was reviewing the situation week by week.

Trouble in the past

But both nations have intervened heavily in grain markets in the past, leading the price hikes and shortages. In 2010, Russia banned wheat exports after drought and wildfires that destroyed 53 million acres of crops. In 2007 and 2008, Russia imposed an export tax on wheat. Ukraine implemented quotas from 2006 to 2008 and again from 2010 to 2011.

High global food prices in 2008 sparked food riots in 48 countries according to a 2011 World Food Program paper. In 2011 food price spikes exacerbated hunger in East Africa where Somalia faced a famine that killed 260,000 according to the United Nations, nearly half of them children.

On March 20, after panic buying of buckwheat in Russia, the government announced a 10-day ban on all exports of processed grains, only to cancel the order four days later. It then announced a 7.7 million-ton quota on grain exports from April to June. The amount, however, does not appear to be a significant reduction based on Russia’s annual export outputs, which includes wheat exports of about 35 million tons.

The Eurasian Economic Union (Russia, Kazakhstan, Belarus, Armenia and Kyrgyzstan) banned exports of sunflower seeds, onions and vegetables outside the union.

In Ukraine, President Volodymyr Zelensky announced restrictions on food exports on March 16 without offering details, setting off alarm.

But the measures had a limited impact: a ban on the buckwheat exports, not a major export, and a wheat export quota of 22 million tons in 2019-20 that was in line with expected exports.

Alex Lissitsa, president of the Ukrainian Agribusiness Club, said Zelensky seemed to be trying to reassure Ukrainians that he was keeping prices down.

“There's no reason to ban the export of wheat right now. . . . Ukraine has an oversupply of many products,” Lissitsa said. “We need to export. Otherwise, we'll actually die with our products here.”

Kazakhstan’s wheat farmers and flour millers see the risk of losing years of hard work cultivating markets in China and Central Asia.

Pyotr Svoik, an independent economist based in Almaty, said the Kazakh government was trying to maintain low domestic prices for socially important goods like bread and flour.

“Kazakhstan produces considerably more wheat and flour than it consumes. Therefore, exports are necessary, and now this issue is especially acute, amid falling oil prices, on which the country's economy depends,” he said.

Shibani Mahtani in Hong Kong contributed to this report. Stern reported from Kyiv and Kumenov from Almaty, Kazakhstan.