"What you're describing appears to be either collective punishment or collective extortion," State Department spokesman Ned Price told reporters.

"It is not an issue between any two countries on the face of the earth. This is an urgent imperative" for developing countries, he said.

"Moscow's suspension of the initiative would be tantamount to collective punishment for the rest of the world, but especially lower- and middle-income countries that so desperately need this grain."

Price voiced alarm that global food prices have already risen because of uncertainty over the grain initiative, which had been negotiated by the United Nations and Turkey.