The US has formally agreed a free trade pact with El Salvador but has told five more Central American nations that they must do more to finalise similar deals.
Vociferous critics of Cafta still remain
Congress sanctioned a Central American Free Trade Agreement (Cafta) last year but official implementation has been delayed by a series of legal wrangles.
The US complained that Cafta partners were failing to harmonise key laws and regulations, as obliged by the treaty.
But the US-El Salvador agreement will now come into force on 1 March.
The announcement, by the US Trade Representative's office, came ahead of a meeting between US President George W Bush and his Salvadorean counterpart Antonio Saca.
Washington has urged Costa Rica, Guatemala, Honduras, Nicaragua and the Dominican Republic to make greater efforts to bring domestic regulations into line with multilateral standards required by the treaty.
Rules governing meat inspection remain a major sticking point.
The US views certain countries' reluctance to recognise its own system as equivalent to their own as a barrier to its exports.
Critics of Cafta - designed to reduce trade barriers between the US and Central America - have claimed that the US has made unreasonable demands in certain areas.
"We hope and expect that we will be able to bring additional Cafta partners into the agreement soon," Rob Portman, the US Trade Representative said on Friday.
Costa Rica has yet to ratify the agreement.