ROME —  U.N. officials said Tuesday they are exploring ways to step up the production of a vaccine in case bird flu mutates and sparks a human flu pandemic (search).

David Nabarro, the U.N. coordinator for avian and human influenza, said it will take six months to build up a stockpile of vaccines, but health authorities are worried that amount of time might be too long if a pandemic flu strain emerges.

"We will need to have vaccines much more quickly than six months," Nabarro said, adding that the World Health Organization (search) and governments are exploring how to "pull together vaccine manufacturers" to see if it can be done more quickly.

"The World Health Organization as we speak is looking at options to get a scaling up of vaccine production capacity," Nabarro said.

For the virus to become a global threat, it would have to mutate into a form that spreads easily from person to person. Efforts to create an effective vaccine are limited until that strain emerges.

However, some countries are preparing a vaccine based on the most common bird flu virus circulating, partly to help them speed up the research so that a human vaccine can be produced more quickly and in larger quantities once the strain emerges.

Last week, U.S. Health and Human Services Secretary Mike Leavitt said "no one in the world is ready" for a catastrophic outbreak of bird flu and President Bush summoned vaccine manufacturers to the White House to discuss the situation.

Speaking Tuesday on NBC's "Today" show, Bush expressed confidence that the government would develop a plan "to handle a major outbreak" if bird flu spreads to the United States.

In Rome, U.N. experts briefed journalists at the U.N. Food and Agriculture Organization about the global bird influenza situation.

Turkey and Romania culled thousands of birds Monday as a precaution after suspecting bird flu in their flocks.

There is no confirmation that birds in Romania and Bulgaria, which is located on the Black Sea between Romania and Turkey, have been infected, U.N. officials said.

Samples from Turkey are on their way to London for definitive testing and results are expected in a few days.

Louise Fresco, an agriculture official at the Rome-based Food and Agriculture Organization, said that the U.N. agencies would be seeking $175 million — "a fraction of what is needed" — for initial efforts aimed at combatting the problem.

Among the first to pledge funds, Fresco said, have been Japan, Switzerland, Germany, the Netherlands, the United States, Australia and the European Union.

It is rare for people to contract bird flu, and most cases have occurred in poultry workers.

Since the outbreak began in 2003 in Asia, the virus has infected 117 people and killed 60, according to the official toll kept by the World Health Organization.

"Once it affects humans it has a very high mortality rate of 50 percent," said Samuel C. Jutzi, director of animal production and health division at the Food and Agriculture Organization.

Assuming a limited supply of vaccines is available, authorities would have to ration who receives them, with those on the front lines of combatting any pandemic likely to get the priority.

But Nabarro raised the question of how to define "front line," saying that in addition to health care workers, it could also mean those helping to keep basic functions going for society and the economy, such as those power and water supplies.