Several MEPs had requested a rethink of the US visa policy towards European states, so that all EU citizens are treated equally. Citizens from Poland, Greece, Cyprus, Bulgaria and Romania are required to apply for a tourist visa if they want to travel to the US, while citizens from all other member states do not have to.

But Ms Napolitano warned the EU legislature to "be careful what it asks for," as there are numerous voices in Congress that want to scrap the visa waiver scheme altogether, citing security concerns.

"We certainly share the view that visa waivers are very important parts of our relationship, but there is a possibility that the entire programme will be subject to a legislative challenge," she said.

A chance for US lawmakers who want the scheme scrapped will occur at the end of this year, when a review of the so-called exit programme is due. Congress has linked lifting the visa requirements for further countries to the setting up of this system which screens and fingerprints people as they leave the US.

Ms Napolitano admitted that the "exit programme" has an important impact on the visa waiver scheme and promised to hold an "intensive dialogue" in the coming months with the European partners.

British Liberal MEP Sarah Ludford said that if the exit programme is not given a green light on cost grounds, "we don't know what will happen to the remaining five EU countries for which the visa regime has not been lifted yet."

"The US administration would need to go back to Congress and say the pillars on which the visa waiver legislation stands can't exist. It won't be the European's responsibility that the criteria for lifting the visa requirements are not met," she told this website.

Speaking about those EU countries which do not need visas, but have to share passenger data with the US administration, Ms Napolitano said she was in favour of reviewing the agreements with eleven EU states on sharing passengers data.

She reassured MEPs that the data was only being used in investigations of terrorism or other serious international crimes.

Tourist tax

Another area of disagreement between EU lawmakers and the American official was the upcoming 'ten-dollar tax' to be levied on every tourist travelling to the US starting next year. The tax is necessary to fund a non-profit company aimed at promoting tourism, she said.

"Unlike some of the European countries, the US do not have in place a separate agency to promote tourism and travel. In these days of reduced budgets, this is the only way to fund that," Ms Napolitano argued.

It is very likely that Mr Obama would approve the bill, despite threats from the European side to introduce "reciprocal" measures.

The tourism-promotion bill was introduced in the US Senate by the Democratic majority leader Harry Reid, who next year stands for a tight re-election in the state of Nevada, home of US tourism and gambling mecca Las Vegas.

As Mr Reid is seen as a key figure for passing important legislation on health care and climate change, the Obama administration is not moving against the tourist tax, despite criticism from international partners.