The World Health Organization is working closely with Mexican officials to make sure effective drugs are available to fight the outbreak of swine flu, but cautions other countries may need more as well if the disease spreads, a senior official said Monday.

Mexican official told WHO after the outbreak of the new strain of virus last week that they have enough antiviral drugs in storage to treat more than 1 million people, but that they realized a rapid spread of the disease could exhaust the stockpile, said WHO Assistant Director-General Keiji Fukuda.

"We are keeping in close contact with the Mexican authorities about their need for antiviral drugs," Fukuda told a telephone news conference. "If the situation does escalate, however, it is clear that other countries in addition to Mexico will also need antiviral drugs."

Swiss drug company Roche Holding AG said WHO has enough of the firm's anti-flu Tamiflu stockpiled to treat up to 5 million people who come down with the new strain of swine flu.

"That is on 24 hours standby," said Roche spokeswoman Martina Rupp.

In addition, she said, Roche can quickly increase production, Rupp said.

Fukuda said WHO is working closely with the manufacturers about keeping the drugs available.

Another manufacturer, GlaxoSmithKline PLC, also has stepped up production of its antiviral drug Relenza, which is similar to Tamiflu, company officials said.

Three-fifths of the WHO stockpiles of Tamiflu are held at Roche facilities in the United States and Switzerland and the remaining amount is stored by WHO at different locations around the world, Rupp said.

The stockpiles held at Roche have been donated to WHO and they can be shipped as soon as WHO gives the word, Rupp said.

"They are actually for those countries that are not so well prepared yet," Rupp said.

The company based in Basel, Switzerland, can ramp up production to make 400 million treatments a year, and already has key ingredients ready, she said. Roche has agreements with drug makers in South Africa, India and China that allow for more production, but she said she was unable to give figures for production under sublicense agreements.

Britain and Japan have recently given new orders for Tamiflu because they are doubling their stockpiles, Rupp said.

Between 85 and 95 countries have created stockpiles of the drug, she said.

Roche began increasing production of Tamiflu in 2005 after the outbreak of bird flu in Asia, but Rupp said it began cutting back in 2007 as government stockpiles filled up.

Tamiflu has been regarded as a first defense against an influenza pandemic since it was shown to be effective against bird flu. WHO has said it also is effective against the new strain of swine flu.

A full treatment takes five days — 10 capsules taken two a day starting within 48 hours of the onset of symptoms.

Countries are rushing to develop contingency plans in case the suspected swine flu being blamed in widespread infections and 149 deaths in Mexico spreads rapidly around the world.

The company built up the capacity to make 4 billion capsules a year, or 400 million treatments, Rupp said.

"It was a capacity that was never used because demand was just not that high," she said. "It's obvious. You cannot keep that up and not use it. So we announced that we are going to scale it down."

At the same time Roche said it would keep enough ingredients on hand to expedite the ramp up of manufacturing if needed, said Rupp.

She noted that WHO said in 2005 that stockpiling in advance is the only way to ensure sufficient supplies are available at the start of a pandemic and that anti-viral medicine is just part of the overall plan that countries have to have in place.

Roche shares rose 4.3 percent to 145.60 Swiss francs ($127.88) on the Zurich exchange Monday.