Trade ministers from the Americas have backed the outline of a deal to create a free-trade zone stretching from Alaska to the southern tip of Chile.
The summit was marred by clashes with protesters
The Free Trade Agreement of the Americas - due to come into effect in 2005 - is a key US economic goal.
A summit on the issue has been taking place all week in Miami, Florida.
There had been fears the talks might collapse amid disagreements between Brazil and the US but the deal actually came a day earlier than expected.
But the trade ministers of the 34 states involved agreed to a proposal announced by the US and Brazilian delegations earlier in the week which allows countries to opt in or out of certain clauses.
The FTAA will cover 800 million people with an output of $14 trillion a year.
"We still have a lot of substantial issues to discuss and they are not easy," said Brazilian Foreign Minister Celso Amorim.
"But what we decided was to pursue a course that will make it possible, creates the conditions to come to an agreement. It doesn't guarantee an agreement, but it creates the condition for an agreement."
Brazil and the US drew up their "menu" approach to the FTAA as a way of overcoming their deep disagreements on how comprehensive the accord should be.
All the Americas except Cuba are at the summit
This went against what many of the other countries meeting in Miami wanted - that is, an all-embracing deal dropping tariffs and trade barriers across the twin continents.
However, America's farmers, not to mention powerful voices in the unions, wanted to limit the agreement's scope and US President George W Bush was reluctant to push the issue with an election due in less than a year.
The location of the talks, in the key electoral battleground of Florida, highlights the importance of the thousands of US citrus farmers whose livelihoods could be threatened if they lose their lucrative subsidies.
In any case, the Bush administration's early enthusiasm for the FTAA has waned as its War on Terror and particularly the situation in Iraq has dominated its attention.
Brazil, for its part, wanted farm access badly and was a leader of the developing-nation moves at the global trade talks in Cancun in September, which blocked US and European attempts to push forward their own agenda.
But it was prepared to leave farm talks for a fresh global round as long as the US did not push too hard for access to Brazil's financial services and technology markets.
Anti-globalisation protesters clashed with police in Miami on Thursday during the summit.
At least three people were arrested as riot police used tear gas to disperse a crowd in front of the hotel where the ministers had gathered.
While ministers from 34 of the 35 countries in the Americas - all except Cuba - debated the proposal at the Intercontinental Hotel, the protesters were marching outside.
Police estimated the number of demonstrators at about 1,000 and described them as well organised.