'From what we understand in Mexico, I think people need to be ready for the idea that we could see more severe cases in this country and possibly deaths,' Richard Besser, acting head of the US Centers for Disease Control and Prevention, told ABC television.

'I am not reassured that we have not seen more severe cases in this country,' he added. 'That's something that people have to be ready for. And we're looking for that.'

US authorities have declared a public health emergency to battle the outbreak, but President Barack Obama, who is closely monitoring the situation, on Monday urged Americans to remain calm saying the illness was a 'cause for concern' but not alarm.

After Mexico, the United States has so far recorded the highest number of cases, with 20 confirmed swine flu patients in five states - eight in New York, seven in California, two in Texas, two in Kansas and one in Ohio. But most of the US victims have recovered fully and have experienced only mild symptoms.

In Mexico there have been 103 confirmed or suspected deaths from swine flu, while the number of cases under observation has topped 1,600.

Health officials warned the situation in the United States could worsen quickly and dramatically. Mr Obama is said to be monitoring the swine flu outbreak closely and has ordered a 'very active, aggressive, and coordinated response.'

But top US disease control officials on Monday rejected as unwarranted an EU warning that appeared to call on Europeans to avoid travel to the United States, Mexico and other areas hit by the outbreak.

'Personally, I would try to avoid non-essential travel to the areas which are reported to be in the center of the cluster,' said EU health commissioner Androulla Vassiliou Monday.

Mr Besser, speaking on CNN on Monday, said the EU reaction was excessive. 'At this point, I would not put out a travel restriction or recommendation against coming to the United States,' he said. -- AFP