The deal, brokered by the UN and Turkey last year, expired on Monday and Russia elected not to renew it for another 60 days, meaning Ukraine’s ability to export grain to much of the world is in jeopardy.
The besieged country is one of the world's largest grain exporters and the move sent wheat prices dramatically up.
Russia “can't get their way, so they just keep threatening the world and the world is suffering,” said Ms Thomas-Greenfield in Colorado on Tuesday night.
“Food prices are going up, there are shortages and it's being impacted more in the global South than anywhere else in the world.”
The deal had provided safe shipping corridors in the Black Sea that allowed Ukraine to ship about 33 million tonnes of food for export, a majority of which went to developing countries.
Ms Thomas-Greenfield said that “millions of people are going to suffer” because of Russia’s actions.
The career diplomat's comments follow Ukrainian Foreign Minister Dmytro Kuleba’s pleas to the international community to Russia’s decision to abandon the deal.
Ms Thomas-Greenfield was speaking at the opening of the Aspen Security Summit, a three-day conference held in the pristine Rocky Mountains that attracts some of the biggest names in US intelligence and security.
This year’s event will be headlined by US Secretary of State Antony Blinken and White House National Security Advisor Jake Sullivan.
The event is held in the exclusive Colorado ski town that is more commonly associated with extreme luxury and outdoor adventures than global affairs.
But organisers insist the dramatic setting, a world away from Washington's stifling summer humidity, is an opportunity for thoughtful ruminations on the state of the world and how the US can chart its path forward.