The World Health Organization (WHO) has confirmed that the swine flu outbreak is now a global pandemic.
Director general of the WHO, Dr Margaret Chan, warned that the world should "brace" itself for more deaths.
She described the current pandemic - the first in more than 40 years - as "moderately severe", but said that the organisation did not anticipate a sudden leap in the death toll.
Across the world, 144 people have died from swine flu, while 28,774 cases have been detected in 74 countries
At a press conference in Geneva, Dr Chan explained: "On the basis of available evidence and this expert assessment of the evidence, the scientific criteria for an influenza pandemic has been met.
"I have therefore decided to raise the influenza pandemic alert from phase five to phase six.
Dr Chan warned that the virus was "unpredictable" and "unstoppable".
An important message to all countries was that they must "maintain continuous vigilance", she said, warning those that thought they were through the first wave of flu to prepare for a possible second wave.
But she added that the virus was under "constant and careful" watch around the world.