The pact, announced by a group of Democratic and Republican senators, would provide a path to permanent residency and citizenship for the vast majority of illegal workers, establish a merit-based points system for future immigrants, and set up a low-wage temporary worker programme.

The bill would also provide 18,000 more Border Patrol agents, and scores of radar and camera towers on the US border with Mexico.

The measure, which emerged from exhaustive round-the-clock talks, is aimed at defusing a fiercely polarizing issue and fast-tracking reform before US political life is consumed by the 2008 congressional and presidential campaign cycle.

The bill still needs to be approved by the entire Senate, where debate is to begin Monday, and the House of Representatives. Approval in either chamber is still uncertain, though it has strong support from President George W Bush.

Some Democratic leaders in charge of pushing the deal through Congress have expressed reservations, even as they forged the legislation together with Republican legislators.

And Republican presidential contender Mitt Romney blasted the bill as an "amnesty," raising one of the issues that blocked similar legislation last year.

Bush, whose staff was deeply involved in crafting the bill, lauded the deal they reached.

"I really am anxious to sign a comprehensive immigration bill as soon as I possibly can," he said. "Today we took a good step toward that direction."

Earlier, Democratic Senator Edward Kennedy said the legislation was true to America's "humanity and our tradition of a nation of immigrants."

"Politics is the art of the possible and the agreement we just reached is the best possible chance we will have in years to secure our borders, bring millions of people out of the shadows and into the sunshine of America."

But Senate Democratic majority leader Harry Reid expressed his reservations, saying he especially worried about "the structure of the temporary worker programme and undue limitations on family immigration."

Nancy Pelosi, the Democratic speaker of the House of Representatives, echoed Reid's sentiments.

"While the bipartisan Senate agreement starts the process, I have serious concerns about some elements of this proposal -- the bill must be improved in the Senate," she said.

Democratic presidential candidate Senator Hillary Clinton said she would scrutinize the pact to see if it honoured America's "proud immigrant heritage."

Republican and Democratic aides said the compromise was aimed at moving US immigration rules to a merit-based system, rather than prioritising claims for extended immigrant families.

"It is intended to reflect the labour needs of the United States in the 21st century, rather than the 19th century," a senior Republican staffer said on condition of anonymity.

If approved, the bill would fund building 321km of barriers and 595km of fencing, and the construction of 70 ground-based radar and camera towers on the Mexican border.

Enforcement measures would have to be in place before the new temporary worker programme and arrangements for those living in the United States illegally can begin.

Those illegally in the country before January 1, 2007 must pay a 5,000 dollar fine to get a non-immigrant 'Z' visa which will allow them eventually to become eligible for a green card or permanent residency.

But the new rules would give more credit for earning a green card to those applicants with English proficiency, advanced education, science, technology and mathematics skills, and other special expertise. In the past priority has gone to people with family members already in the United States legally.

The deal will also raise the number of H-1B visas, used by US universities and companies to find skilled workers like scientists, medical experts and computer programmers.

Former Massachusetts governor Romney, who is seeking the presidency in 2008, was quick to criticize the deal.

"It is the wrong approach," he said in a statement. "Any legislation that allows illegal immigrants to stay in the country indefinitely ... is a form of amnesty."

Immigration is a red-hot political issue among core Republican Party supporters, who overwhelmingly favour the strictest controls possible.