Public buildings have closed across Mexico City because of the outbreak
World health experts are investigating a new strain of flu that may have killed as many as 60 people in Mexico.
The US Centre for Disease Control (CDC) said tests so far seem to link Mexico's outbreak with a swine flu virus that had sickened eight in the southern US.
The CDC said it was taking the virus - about which it was working to learn as much as possible - seriously.
Mexican authorities have closed public buildings, suspended public events and launched a vaccination campaign.
It is suspected the virus may have been involved in the deaths of about 60 people, mostly in and around Mexico City, since mid-March.
A new swine flu strain has been confirmed in 16 of the deaths and 44 others are being tested, Mexico's health secretary said. More than 900 other people are thought to have been infected.
Officials said most of those killed so far were young adults - rather than more vulnerable children and the elderly.
Schools, museums and libraries have been closed across the capital region and people are being urged to avoid shaking hands or sharing crockery. All public events have been suspended, an official said.
In Geneva, the World Health Organization (WHO) said an emergency committee would likely convene over the weekend.
And in the US, the White House said it was watching the situation.
Dr Richard Besser, acting director of the CDC, said that preliminary tests on seven out of 14 samples from patients in Mexico had matched the virus found in the US.
Eight people have fallen sick - six in California and two in Texas - with a virus that experts say is a new strain of swine flu. All eight have recovered and only one was hospitalised.
Swine flu is a respiratory disease found in pigs
Human cases usually occur in those who have contact with pigs
Human-to-human transmission is rare and such cases are closely monitored
Dr Besser said more information was needed to determine the extent of the threat posed by the virus. It was unclear, he said, why it was proving fatal in Mexico but not in the US.
"We are not at the point - WHO is not at the point - of declaring a pandemic; we are at the point of trying to learn more about this virus, understand its transmission, and how to control it," he said.
"In Mexico, other influenza viruses are circulating there, so sorting out which cases are caused by swine flu, which are other viruses and which are co-infections will be very important public health information."
The WHO said it had prepared "rapid containment measures" in case they were needed.
But both it and the CDC said that there was no need at this point to issue travel advisories for parts of Mexico or the US.
The CDC said it planned to send experts to Mexico to help investigate the virus.
Swine flu is a respiratory disease which infects pigs. It does not normally infect humans, although sporadic cases do occur usually in people who have had close contact with pigs.
There have also been rare documented cases of humans passing the infection to other humans.
Such cases are monitored very closely because of fears that a new strain of swine flu with the ability to pass from person to person could spark a pandemic.