The revelations came on the same day that France's health minister reported the country's first two confirmed cases of the virus and Denmark confirmed that one person had been infected.
Germany earlier reported that a nurse living in Bavaria had been infected with the virus after contracting it from a patient who had been to Mexico.
German officials said the nurse has since recovered.
Germany's discovery is the second case of human-to-human transmission within a country other than Mexico and it has raised the spectre of a wider epidemic.
Spain reported a case of human-to-human transmission to a person who has not visited Mexico.
The person had contracted the flu from a partner who had travelled to the Latin American country.
Tests are also under way in the UK on a possible human-to-human transmission.
Al Jazeera's Jonah Hull, in Geneva, said that the human-to-human transmission of the disease was crucial in whether the virus would spread widely.
"That raises the possibility at least that human-to-human transmission is taking place on a sustained basis away from the epicentre," he said.
"And that leads to the question of whether number six on the pandemic scale is called by the WHO [the World Health Organisation]. Whether it is, in fact, then defined as a real pandemic."
Level six is defined as human-to-human sustained transmission in at least two regions or continents of the world and is the highest of the WHO's rankings.
The alert level now stands at five.
Hull reported: "We already have Mexico and the US. Now we have Europe and the possibility that this is not only connected to those who have been in Mexico. That is key to the progression of this whole thing towards pandemic status."
Meanwhile, Mexico began on Friday a five-day shutdown of the country in an attempt to halt the spread of the flu. The country said 16 people had died from the flu and another 328 were infected.
The WHO said on Friday that tests had shown that the seasonal vaccine against flu would have little effect against the new H1N1 strain.
Sixteen confirmed in Mexico out of 159 suspected. One death in the United States
Countries with confirmed cases: Mexico, the US, Canada, New Zealand, Spain, Israel, Britain, Germany, Austria, Switzerland, The Netherlands, China, Denmark, France, South Korea
Countries with suspected cases: Australia, Brazil, Chile, Colombia, Norway, Guatemala
Annual influenza epidemics are thought to result in three to five million cases of severe illness and between 250,000 and 500,000 deaths around the world, WHO says
The organisation has rejected accusations that it responded too slowly to the epidemic when it was initially notified of its potential early on April 24.
"There are cases of influenza all the time, but once we knew that this illness was caused by a new influenza virus ... we moved into operation within a matter of hours," Thomas Abrahams, a WHO spokesman, said.
"One of the things we are doing internally is documenting everything we have done, when we did it and how we have done it," he said.
Worldwide there are 17 confirmed deaths caused by the H1N1 virus - one in the US and the rest in Mexico.
Hull said that the number of confirmed cases was "rising fairly rapidly."
"But it is still a figure that is quite well below the individual national figures that are being reported by health ministries around the world, which put the number of confirmed cases at around 500.
"That's because those cases are then vetted by the WHO so that the WHO can in its own time independently confirm."
Dr John Simons, a consultant in infectious disease and tropical medicine, told Al Jazeera from Hong Kong: "It is a new flu virus and it has the potential of causing a pandemic.
"All of the precautions taken so far have been very justifiable and we need to be prepared for a pandemic.
"However, having said that, we need to look at this with some sort of perspective.
"Firstly, ordinary seasonal flu kills between 200,000 and 500,000 people throughout the world every year, and in the United States 36,000 people.
"So far, this new H1N1 virus is appearing to act rather like ordinary flu.
"The difference is, however, that this is a new virus and nobody has any immunity to it.
"So it is likely to sweep around the world and probably will cause a pandemic. And will cause significant morbidity and some degree of mortality."