UN chief Antonio Guterres warned Wednesday of years of mass hunger and famine if a growing global food crisis goes unchecked as he urged Russia to release Ukrainian grain.

Speaking at a major United Nations summit in New York, Guterres said the war in Ukraine was compounding global food insecurity already worsened by warming temperatures and the coronavirus pandemic.

Guterres said that in just two years, the number of severely food insecure people has doubled -- from 135 million pre-pandemic to 276 million today.

He added that more than half a million people are living in famine conditions, an increase of more than 500 percent since 2016.

"Now the war in Ukraine is amplifying and accelerating all these factors: climate change, Covid-19 and inequality," he told the meeting on the growing food crisis chaired by US Secretary of State Antony Blinken.

"It threatens to tip tens of millions of people over the edge into food insecurity, followed by malnutrition, mass hunger and famine, in a crisis that could last for years," Guterres added.

Before Russia's invasion of its neighbor in February, Ukraine was seen as the world's bread basket, exporting 4.5 million tonnes of agricultural produce per month through its ports -- 12 percent of the planet's wheat, 15 percent of its corn and half of its sunflower oil.

But with the ports of Odessa, Chornomorsk and others cut off from the world by Russian warships, the supply can only travel on congested land routes that are far less efficient.

Guterres called on Russia to free up Ukrainian exports of grain.  "Let's be clear: there is no effective solution to the food crisis without reintegrating Ukraine's food production," he said.

"Russia must permit the safe and secure export of grain stored in Ukrainian ports. "Alternative transportation routes can be explored -- even if we know that by itself, this will not be enough to solve the problem," Guterres added.

The UN chief also said that Russian food and fertilizers "must have full and unrestricted access to world markets." Russia is the world's top supplier of key fertilizers and gas.

The war and international economic sanctions on Moscow have disrupted supplies of fertilizer, wheat and other commodities from both countries, pushing up prices for food and fuel, especially in developing nations.

The fertilizers are not subject to the Western sanctions but sales have been disrupted by measures taken against the Russian financial system, diplomats say.