(CNN) -- The number of confirmed swine flu cases worldwide rose to 154, with six additional cases reported in Spain, the World Health Organization said Thursday.
Quarantine officers monitor arrivals with a thermographic device at Bangkok's main international airport.
Until now, the country had four confirmed cases.
Of the 10 cases of the H1N1 virus in Spain, nine were found in people who had returned from Mexico.
But authorities are troubled about the 10th case which they say is a case of community transmission, spread from one person in the community to another, said WHO spokesman Dick Thompson.
If Spain now sees more such cases of community transmission, the world body may have to elevate its pandemic alert another notch to its highest level.
Phase 6 is the pandemic phase and is characterized by a community-level outbreak in another country in a different WHO region.
Spain falls in another region than the United States and Mexico -- the two countries that have until now shown human-to-human spread.
"The significance is that it's another phase," Thompson said. "The virus is becoming established in another area, another country."
But, Thompson added, the virus would have to show a sustained pattern of transmission in order for the level to be raised.
"We're looking for intergenerational spread. So, that means from a traveler to a spouse to the butcher to the kindergarten, something like that -- through generations of spread in one community."
An estimated 2,700 people are suspected of suffering from the virus worldwide. The first cases were detected in Mexico, where health officials suspect swine flu in more than 150 other deaths and roughly 2,500 illnesses.
As of late Wednesday, 99 cases had been confirmed -- up from 26, Mexico's health secretary reported. An eighth fatality was also confirmed. However, the additional cases and fatality were not confirmed by the WHO.
For now, the WHO's breakdown of confirmed cases is:
-- United States: 91, including one death
-- Mexico: 26, with seven deaths
-- Canada: 13
-- Spain: 10, including one suspected case of community transmission
-- United Kingdom: 5
-- Germany: 3
-- New Zealand: 3
-- Israel: 2
-- Austria: 1
Elsewhere, Peru, Switzerland and The Netherlands reported their first cases late Wednesday and early Thursday respectively -- but they were not among the WHO's official tally.
The WHO is also investigating possible swine flu cases in the following countries:
-- New Zealand: 11
-- France: 2
-- South Korea: 1
-- Switzerland: 1
The world body defines "possible" as cases where the patient tested positive for Influenza A -- the general category of strains that includes the H1N1 swine.
Further tests are needed to verify whether they are positive for the virus itself.