A crucial portion of the world’s wheat, corn and barley is trapped in Russia and Ukraine because of the war, while an even larger portion of the world’s fertilizers is stuck in Russia and Belarus. The result is that global food and fertilizer prices are soaring. Since the invasion last month, wheat prices have increased by 21 percent, barley by 33 percent and some fertilizers by 40 percent.

The upheaval is compounded by major challenges  that were already increasing prices and squeezing supplies, including the pandemic, shipping constraints, high energy costs and recent droughts, floods and fires.

Now economists, aid organizations and government officials are warning of the repercussions: an increase in world hunger.

The looming disaster is laying bare the consequences of a major war in the modern era of globalization. Prices for food, fertilizer, oil, gas and even metals like aluminum, nickel and palladium are all rising fast — and experts expect worse as the effects cascade.

“Ukraine has only compounded a catastrophe on top of a catastrophe,” said David M. Beasley, the executive director of the World Food Program, the United Nations agency that feeds 125 million people a day. “There is no precedent even close to this since World War II.”

Ukrainian farms are about to miss critical planting and harvesting seasons. European fertilizer plants are significantly cutting production because of high energy prices. Farmers from Brazil to Texas are cutting back on fertilizer, threatening the size of the next harvests.

China, facing its worst wheat crop in decades after severe flooding, is planning to buy much more of the world’s dwindling supply. And India, which ordinarily exports a small amount of wheat, has already seen foreign demand more than triple compared with last year.

For those living on the brink of food insecurity, the latest surge in prices could push many over the edge. After remaining mostly flat for five years, hunger rose by about 18 percent during the pandemic to between 720 million and 811 million people. Earlier this month, the United Nations said that the war’s impact on the global food market alone could cause an additional 7.6 million to 13.1 million people to go hungry.

The World Food Program’s costs have already increased by $71 million a month, enough to cut daily rations for 3.8 million people. “We’ll be taking food from the hungry to give to the starving,” Mr. Beasley said.

While virtually every country will face higher prices, some places could struggle to find enough food at all.

Armenia, Mongolia, Kazakhstan and Eritrea have imported virtually all of their wheat from Russia and Ukraine and must find new sources. But they are competing against much larger buyers, including Turkey, Egypt, Bangladesh and Iran, which have obtained more than 60 percent of their wheat from the two warring countries.

“This year’s seedling situation can be said to be the worst in history,” said China’s agriculture minister, Tang Renjian.

Rising food prices have long been a catalyst for social and political upheavals in poor African and Arab countries, and many subsidize staples like bread in efforts to avoid such problems. But their economies and budgets — already strained by the pandemic and high energy costs — are now at risk of buckling under the cost of food, economists said.

Tunisia struggled to pay for some food imports before the war and now is trying to prevent an economic collapse. Inflation has already set off protests in Morocco and is helping stir renewed unrest and violent crackdowns in Sudan.

Countries afflicted by protracted conflict, including Yemen, Syria, South Sudan and Ethiopia, are already facing severe hunger emergencies that experts fear could quickly worsen.

In Afghanistan, aid workers warn that the humanitarian crisis has already been exacerbated by the war in Ukraine, making it more difficult to feed the roughly 23 million Afghans — more than half the population — who do not have enough to eat.

Nooruddin Zaker Ahmadi, the director of Bashir Navid Complex, an Afghan imports company, said that prices were rising across the board. It took him five days in Russia this month to find cooking oil. He bought 15-liter cartons for $30 each and will sell them at the Afghan market for $35. Before the war, he sold them for $23.

For the global food market, there are few worse countries to be in conflict than Russia and Ukraine. Over the past five years, they have together accounted for nearly 30 percent of the exports of the world’s wheat, 17 percent of corn, 32 percent of barley, a crucial source of animal feed, and 75 percent of sunflower seed oil, an important cooking oil in some parts of the world.

There are other hurdles. The United Nations estimated that up to 30 percent of Ukrainian farmland could become a war zone. And with millions of Ukrainians fleeing the country or joining the front lines, far fewer can work the fields.

Russian and Ukrainian wheat is not easily replaced. Inventories are already tight in the United States and Canada, according to the United Nations, while Argentina is limiting exports and Australia is already at full shipping capacity. Over the past year, wheat prices are up 69 percent. Among other major food exports of Russia and Ukraine, corn prices are up 36 percent and barley 82 percent.

The war also threatens another longer-term shock to the food markets: a shortage of fertilizer.

Matt Huie, a farmer near Corpus Christi, Texas, said that skyrocketing prices had already forced him to stop applying fertilizer to the grazing fields that nourish his hundreds of cows, assuring that they will be skinnier come slaughter. Now he is worried he will have to also reduce fertilizer for his next corn crop, which would slash its yield. “We’ve gotten into uncharted territory,” he said.