A summit of more than 30 leaders from across the Americas has opened in the Mexican city of Monterrey.
Bush's attendance has attracted protesters
Attendees will debate a wide range of economic and security matters such as free trade, terrorism and corruption.
Before the summit Mexican President Vicente Fox welcomed US plans to give temporary worker permits to millions of Mexican illegal workers in the US.
But correspondents say the summit could be threatened by clashes between the US and the region's left-wing leaders.
Even before the meeting began, delegations struggled to agree on the agenda and a draft final document.
The United States wants to focus on trade, security and corruption, while Brazil and other countries want to highlight social issues.
US President George W Bush and his Mexican host met privately before the opening of the summit, before holding a joint press conference in which Mr Fox approved Mr Bush's proposal to offer some illegal immigrants in the US the right to work legally.
"This is a very important step forward," Mr Fox said.
Mr Fox accept an invitation to visit to Mr Bush's ranch in Texas in March.
In August 2002 Mr Fox cancelled a visit to the ranch in protest at the US state's execution of a Mexican national who killed a US police officer.
Mr Bush, however, insisted there would not be a blanket amnesty for the millions of illegal Mexican workers in the US.
He said: "I oppose amnesty because it encourages violation of our laws."
The apparent softening of Mr Fox's position regarding the US came only a few hours after he and Canadian Prime Minister Paul Martin appeared unhappy about the US's recently increased border security.
Mr Martin said borders "must be open to our respective populations going back and forth", while Mr Fox stressed that security and economic growth should be given equal importance.
Brazil was angered by increased US security checks for foreign visitors
Both also announced that they favoured strengthening and expanding the existing North American Free Trade Agreement.
Other leaders in Monterrey for the Special Summit of the Americas (SSM) have voiced opposition to some of the US' economic objectives.
While the US wants the summit to re-emphasize a 2005 deadline for finishing talks over the proposed Free Trade Area of the Americas, Brazil and Venezuela say the event is neither the time nor the place to discuss the matter.
Another controversial topic is the fight against corruption.
Washington has called for corrupt governments to be expelled from the Organization of American States (OAS) - a move opposed by some Latin American members, who say it would be impossible to agree on a criteria defining corruption.
And Brazil has also strongly condemned the increased security measures in the United States designed to prevent terror attacks.
It retaliated against the US decision to photograph and fingerprint foreign visitors by doing the same to US citizens arriving in Brazil.
Anti-globalisation and environmental protesters have also arrived in Monterrey, and are expected to hold demonstrations during the summit.