Atta's visa was approved six months after he is believed to have crashed a plane in New York

Officials also want to limit the time granted to business visitors and tourists to 30 days from as much as six months.

The tightening of rules comes after outrage that the Immigration and Naturalization Service issued student visas to two of the 11 September hijackers - six months after the attacks took place.

Under the changes which have already taken effect:

• All foreigners must obtain student visa before they begin studies

• Foreigners may not enter the US on a business or visitor visa and then start studying while awaiting a student visa.

Two of the men who hijacked planes and crashed them in New York and Washington on 11 September came to the US on visitor visas and later applied for student visas.

Approvals of the student permits for Mohammed Atta of Egypt and Marwan al-Shehhi of United Arab Emirates were sent just last month to a flight school in Florida where they are believed to have practised for the attacks.

Bush 'stunned'

All 19 of the suspected hijackers entered the US legally, though three overstayed their visas.

At the time of the attacks, approximately 600,000 foreign students were enrolled in US colleges and universities and INS officials acknowledged they could not verify the whereabouts of many.

News of the visa approvals for Atta and al-Shehhi - believed to be the men who guided two passenger jets into the towers of the World Trade Center - sparked outrage from President George W Bush and Attorney General John Ashcroft.

Mr Bush, who said he was "stunned" by the incident, fumed: "[The INS] needs to be modernised so we know who's coming, and who's going out and why they're here."

These new rules strike the appropriate balance between INS' mission to ensure that our nation's immigration laws are followed and stop illegal immigration and our desire to welcome legitimate visitors

INS Commissioner James Ziglar

The proposed overhaul of visitor visas would eliminate the six-month maximum admission period and visas would be granted for the amount of time needed for a trip.

In practice, that would average about 30 days, the INS said.

Extensions would only be granted if a visitor could show an unexpected or compelling reason, such as the need for medical treatment or a delay in completing a business matter, one official said.

The changes would have a huge impact on people visiting relatives for more than one month and retirees and young people who want to spend many weeks sightseeing.

The INS said about 10 million people received tourist visas in 2000, the latest year with available data, and 75% of them stayed less than a month.

Another 2.5 million travelled to America on business and stayed an average of 13 days.

INS Commissioner James Ziglar said: "These new rules strike the appropriate balance between INS' mission to ensure that our nation's immigration laws are followed and stop illegal immigration and our desire to welcome legitimate visitors to the United States."