Cuba says its future purchases of American food have been threatened by the US government's decision to tighten rules on exports to the island.
US President Bush would like to see Fidel Castro out of power
The US treasury department said on Tuesday that Cuba would have to pay cash before US farm goods were shipped.
As a result, Cuba fears the goods might be seized by US litigants with claims against the Havana government.
Cuba's state agency Alimport says it will honour current US contracts, but may soon have to seek other suppliers.
The agency's head, Pedro Alvarez, told a news conference the move signalled an "escalation" of efforts by Washington to obstruct trade with Cuba.
"Under this procedure, goods earmarked to the Cuban people could apparently be liable to court-ordered seizures in the United States, to satisfy legally-groundless claims against Cuba," he said in a statement.
"To purchase from the US under the new measure would be highly unreliable, for the direct food supplies to the Cuban population, including its children, would be at risk."
Pressure on Castro
Since Congress softened the four-decade-old US trade embargo on Cuba in 2000, Cuba has purchased US goods worth nearly $800m.
But President George W Bush's administration has implemented tougher measures in the past year as part of efforts intended to hasten the fall of Fidel Castro.
Congressmen from farming states in the US criticised the new ruling, with one Democrat senator, Max Baucus of Montana, threatening to block nominees to treasury posts.
"I'm outraged at this attempt by treasury department bureaucrats to choke off US agriculture sales to Cuba," Senator Baucus - the most senior Democrat on the Senate finance committee - was quoted as saying by the Associated Press news agency.
However, treasury department officials said the decision brought the trade into line with international practice.