An epidemic of a swine flu never identified before has broken out in Mexico City killing at least 16 people so far and raising fears of a pandemic.

All schools, museums, libraries and state-run theatres in the metropolitan area were closed today in an attempt to control the spread of the virus that authorities say may be linked to a further 45 deaths.

"This is a new virus that we haven't seen before," health minister Jose Angel Córdova said in an interview with MVS radio. "We have taken these measures because this is a virus that has the potential to become a pandemic."

The authorities say they are investigating close to 1,000 suspicious cases of flu that they are concerned may prove to be caused by the new virus. Most are in the metropolitan area of 20 million people, although three other Mexican states have also been affected to a lesser degree.

There have also been seven reported cases of the same virus reported in the United States, five in Southern California and two in Texas. All those patients have recovered.

The Mexican minister said the authorities were considering extending the precautions to include shutting down workplaces as well as public buildings, but for the moment urged employers to be tolerant of absences.

He said that while the situation was "very worrying" he believed the epidemic "is controllable".

The impact of the preventative measures on city life was felt immediately as the population woke up to news of the epidemic that was announced in a late night statement. Radio and TV stations repeated official advice to stay away from crowded places "unless urgently necessary," and to seek medical help at the first sign of the very high fevers and acute respiratory symptoms associated with the illness.

At Mexico City's biggest airport, airlines began requiring passengers checking in for domestic and international flights to fill out forms to help decide who could be at risk of carrying the virus. Anybody deemed to be so was reportedly asked not to fly.

Meanwhile, the media was flooded with questions from city dwellers concerned about everything from the dangers of eating pork to travel on the metro.

Many people heading to work in the morning wore blue surgical face masks, and chemists said their supplies were running out.

The partial shutting down of the metropolis was a dramatic reversal from the government's previous position that minimised the unusual number of flu deaths picked up by the media, saying it was due to an extension of winter.

Córdova said the sudden change of tack happened when samples analysed in highly specialised laboratories in Canada and the US revealed that the virus causing the deaths was a completely different strain.

The Geneva-based WHO said it was concerned about the epidemic and had activated its Strategic Health Operations Centre. The agency added that it was in daily contact with US, Canadian and Mexican authorities.