By Rob Watson BBC Washington Correspondent

More hi-tech security to stop potential terrorists entering the US along its long border with Mexico has been announced by President George W Bush.

The measures include the use of X-ray machines to examine the contents of lorries crossing the border and the development of new technology to help identify legitimate travellers.

I want this border to be modern, I want it to have the very best technology

George W Bush

The US has already done a deal with Canada to develop permanent resident cards and an immigration database.

Speaking in the Texas border town of El Paso before beginning his four-day visit to Mexico, Peru and El Salvador, Mr Bush praised Mexico as a good and reliable neighbour.

He said that the new hi-tech measures being introduced were not designed to keep out Mexican goods and legitimate visitors.

"On the other hand, we want to use our technology to make sure that we weed out those who we don't want in our country, the terrorists, the smugglers, those who prey on innocent life," he said.

"I want this border to be modern, I want it to have the very best technology."

Illegal immigration across the border has been a problem

With about 300 million people crossing from Mexico into the US every year, Washington regards the border as a weak link in its efforts to prevent further terrorist attacks.

Strengthening the border is likely to feature prominently in talks between President Bush and Mexico's President Vicente Fox.

They have been meeting on the fringes of the UN development summit in the northern Mexican city of Monterrey.

On Thursday, Mr Bush asked Congress for another $5bn to be "spent on our airports and on our borders, to make sure Americans are more secure and more safe than ever before".