UNITED NATIONS (Reuters) - World readiness for an influenza pandemic has improved after an "extraordinary global response" to the bird flu threat of recent years, the top U.N. official dealing with the disease said on Tuesday.

But David Nabarro, the world body's influenza coordinator, said the risk of a pandemic remained, should the bird flu virus mutate into a form that is easily transmitted between humans.

The highly pathogenic H5N1 avian influenza strain has caused the death or destruction of an estimated 300 million birds since it resurfaced in Asia in 2003. While H5N1 rarely infects people, it has killed 241 out of 383 infected in 15 countries.

Worldwide concern about the disease mounted in 2005 as cases were found in Europe, the Middle East and Africa.

Nabarro told a U.N. news conference that "there has been an extraordinary global response to ... the spread of the virus ... that has meant that now the continuous transmission of this virus is only occurring in four, perhaps five countries."

Elsewhere, he said, "the situation is really improving. Countries generally have invested massively in improving the functioning of their veterinary services, and also the security ... around which poultry are reared has generally improved."

He said the United Nations was most concerned about Indonesia, which has the most human cases, but also about Vietnam, Bangladesh and Egypt. The situation in Nigeria had "really calmed down" this year, he added.

He cited South Korea and Britain as examples of countries that responded vigorously to recent bird flu outbreaks. A total of some $2.7 billion was pledged to fight the disease at a series of international conferences.

The world was also much better prepared than it was in 2005 to cope with a possible pandemic, with about 160 nations having plans in place, Nabarro said. The United States, Australia and Singapore ran drills to see how their financial sectors would deal with a bird flu outbreak.

"What these exercises are showing is it's not expensive to prepare for a pandemic and that if you are pandemic-ready, it also helps you to prepare for other disruptions," he said.

But Nabarro, who warned in September 2005 that between 5 million and 150 million people could die in an influenza pandemic that he expected "soon," said on Tuesday he was not saying the threat had receded. The most serious flu pandemic of modern times, in 1918, killed some 40 million people.

The H5N1 virus "can still recrudesce like a forest fire that's smoldering. It could still come back at any time if we don't watch it," he said.

"We are anticipating that there will be another pandemic at some time. ... It could become human-to-human at any time. ... The probability is still there and it hasn't changed."