"I think it would be, at this stage, unwise to suggest that, in any way, those events are out of control or spreading in an uncontrolled fashion," he said. "I think the next few days will tell as this develops."
"At the present time I would still propose that a pandemic is imminent because we see the disease spread," Dr Ryan added.
The WHO is sending 2.4m courses of antiviral treatment to 72 nations around the world, Dr Ryan said, among them many developing countries.
But in cases of the virus outside Mexico, the effects do not appear to be severe.
Italy and the Irish Republic reported their first cases on Saturday, bringing the number of countries affected to 18.
Canada has announced that a herd of pigs has tested positive for swine flu.
A senior agriculture official told a news conference that the pigs may have been infected by a farm worker who fell ill after returning from Mexico last month.
In Egypt, authorities have begun in earnest the slaughter of more than 300,000 pigs, in what was originally described as a precaution against swine flu.
Officials now say the move is a general health measure aimed at restoring order to Egypt's pig-rearing industry.
International experts say there is no scientific rationale for Egypt's pig cull
Experts say the virus cannot be caught from eating pork and there is no scientific rationale for the cull.
Five countries outside Mexico have confirmed person-to-person transmission.
China is trying to stop the spread of the virus, after getting its first case on Friday.
It says it will quarantine all those who travelled on a flight from Mexico with a man suffering from swine flu.
Flights from Mexico have been suspended, and fellow guests and staff at the Hong Kong hotel where he was staying have been quarantined.
On Saturday, Mexico's foreign minister advised citizens not to travel to China to avoid the health measures being taken there against Mexicans.
The US has now confirmed 160 cases of swine flu across 21 states but has seen only one death, of a Mexican toddler in Texas.
SYMPTOMS - WHAT TO DO
Swine flu symptoms are similar to those produced by ordinary seasonal flu - fever, cough, sore throat, body aches, chills and fatigue
If you have flu symptoms and recently visited affected areas of Mexico, you should seek medical advice
If you suspect you are infected, you should stay at home and take advice by telephone initially, in order to minimise the risk of infection
President Barack Obama said in his weekly radio address that the US was taking "all necessary precautions" to ensure it was prepared if the virus developed into "something worse".
Dr Anne Schuchat, acting deputy director of the US Centers for Disease Control and Prevention (CDC), said that although experts were concerned about the possibility of severe cases, the majority so far had been "mild, self-limited illness".
The new virus lacked the traits that made the 1918 flu pandemic so deadly, another CDC official said.
Mr Cordova appeared to agree, saying that the Mexican authorities may, on reflection, have overestimated the danger.
He said 43.7% of samples from suspected cases so far tested had come back positive, a total of 397. Sixteen in this group had died.
"That means that apparently, the rate of attack is not as wide as was thought," he said.
But he stressed that the risk of a rise in infection remains and said some elements of the five-day shutdown - in which many public buildings and businesses have been closed and people urged to stay at home - might be extended.
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