President George W Bush has announced plans to introduce an additional 6,000 guards along the US border with Mexico, in a live televised address.
Mexico is concerned about troop build-up along the border
Mr Bush unveiled a five-point plan for reforms on immigration, an issue he said stirred "intense emotions".
As well as fortifying borders, Mr Bush said he would introduce a temporary guest-worker scheme, and allow some illegal immigrants to become citizens.
The plan met with a sceptical response from Mexico and some border states.
"The president is putting the onus on border governors to work out the details and resolve the problems with this plan," said New Mexico Governor Bill Richardson.
California Governor Arnold Schwarzenegger, one of America's most famous immigrants, endorsed more border security.
But he said he was "concerned asking National Guard troops to guard our nation's border is a Band-Aid solution and not the permanent solution we need".
Reaction among members of Congress, who are debating proposed reforms to immigration laws, also ranged from enthusiastic to lukewarm to outright rejection.
"The decision to send troops is the shot in the arm we need to strengthen our borders and protect our families," said House Speaker Dennis Hastert.
"The National Guard already is stretched to the limit by repeated tours in Iraq and Afghanistan, as well as providing disaster assistance in their own states," Senator Edward Kennedy said.
Analysts say Mr Bush was trying to appeal to opposing sides of the debate, appeasing both grass-roots conservative Republicans and the country's growing number of Hispanics.
There are an estimated 11.5 million illegal immigrants in the US, about half of them of Mexican origin.
US 'not in control'
Mr Bush's live prime-time address began with a hiccup when part of a last-minute rehearsal was accidentally broadcast.
US ILLEGAL IMMIGRANTS
About 11.5m illegal immigrants in the US
Four out of 10 have been in US five years or less
75% were born in Latin America
Most enter via southern US border
California, Texas and Florida host most illegal immigrants
Many work in agriculture, transport and construction
Once the speech was under way, Mr Bush said the country had in recent weeks seen the emotions stirred by immigration put on display as millions of immigrants and supporters took to the street to protest against plans to criminalise illegal immigrants.
He said the US had not been "in full control of its borders", allowing illegal migrants to "sneak across, and millions have stayed".
But he said the vast majority were "decent people who work hard, support their families, practise their faith, and lead responsible lives".
He said his reform objectives included:
- Securing US borders: The number of border guards would rise by 6,000 to 18,000, and new hi-tech detection measures including hi-tech fences, new border patrol roads, and motion sensors would be installed along the 2,000-mile (3,200km) southern border.
- A temporary worker programme which would match "honest immigrants" with jobs that employers could not find Americans to do. The workers would have to return to their country of origin after a specified time.
- New ID cards for legal foreign workers, to include biometric technology, would allow employers to verify they were hiring legal workers.
- Providing illegal immigrants with a chance of citizenship. Mr Bush said applicants would have to pay a penalty for breaking the law, pay back taxes, learn English, and would have to "wait in line" behind legal migrants.
- Encouraging immigrants to assimilate, to learn English "and embrace our common identity as Americans".
On border security, Mr Bush said 6,000 National Guards would be sent to the southern border for one year while extra border guards were trained, but that they would not undertake direct law enforcement.
Detention facilities would be increased, and legal procedures speeded up, so that illegal immigrants could be quickly deported.
On allowing long-term illegal immigrants a "path to citizenship", Mr Bush said this was "not an amnesty" but a "rational middle ground between granting an automatic path to citizenship for every illegal immigrant and a programme of mass deportation".
In his address, Mr Bush referred to concerns raised by Mexico's President Vicente Fox, saying the US "is not going to militarise the southern border".
But Mexico's foreign ministry expressed doubt in a statement, saying: "We have to express our concern that these actions are not accompanied by enough progress in the legislative process."
Mr Bush appealed to Congress to help him pass his plans into law. The whole two-year package will cost about $1.9 billion.
The immigration issue has sparked fierce debate in the US and is high on the agenda as Republicans seek to retain control of Congress in November's mid-term elections. Some lawmakers have questioned whether the deployment of the National Guard would overstretch the US military.
US-MEXICO - WORLD'S BUSIEST BORDER
3,200km (2,000 miles) border divided into nine Border Patrol sectors
US plans 1,130km (700 mile) fence along part of border
Border guards made 1m arrests in 2005
500 people died trying to cross illegally in 2005, say rights groups
Thousands cross legally daily to shop and work - in 2003 88m cars, 48m pedestrians crossed