Mexican authorities have closed schools and public buildings in the capital in a bid to contain a new flu virus suspected of killing up to 60 people.
Public events were suspended and residents donned face masks as concern grew over the outbreak.
Health experts say tests so far seem to link it with a new swine flu virus that sickened eight in the southern US.
The World Health Organization's head flew urgently to Geneva to oversee the agency's handling of the outbreak.
Margaret Chan curtailed her visit to Washington to return to WHO's headquarters where an emergency committee would likely convene in the coming hours.
The WHO said it had prepared "rapid containment measures" in case they were needed.
"We are very, very concerned," said WHO spokesman Thomas Abraham.
"We have what appears to be a novel virus and it has spread from human to human. It's all hands on deck at the moment."
The WHO is also sending its top experts to Mexico and the US to work with the local officials.