The virus was first recorded in Mexico
The global number of swine flu deaths has jumped by more than 1,000 in a week, latest figures from the World Health Organization (WHO) show.
At least 7,826 people are now known to have died following infection with the H1N1 virus since it first emerged in Mexico in April.
Europe saw an 85% increase in the week, with the total number of deaths rising from at least 350 to at least 650.
However, in most cases the virus continues to produce mild symptoms.
An overwhelming majority of patients usually recover, even without medical treatment, within a week.
The biggest rise in deaths was recorded in the Americas, where the death toll rose to 5,360 - a rise of 554 cases in one week.
Health authorities in Norway and France have each recorded two fatalities from a mutated strain of H1N1.
China, Japan, Norway, Ukraine and the US have also recorded cases of people being infected with a mutated strain.
French health officials confirmed that two patients infected by a mutation that was also recently detected in Norway had died in two different cities in France.
SWINE FLU SYMPTOMS
1. High temperature, tiredness and lowered immunity
2. Headache, runny nose and sneezing
3. Sore throat
4. Shortness of breath
5. Loss of appetite, vomiting and diarrhoea
6. Aching muscles, limb and joint pain
"This mutation could increase the ability of the virus to affect the respiratory tracts and, in particular, the lung tissue," said a statement from the government's Health Surveillance Institute.
The French institute added that, in the case of one of the patients who died, the mutation was accompanied by another mutation known to confer resistance to the main drug being used to treat swine flu, which is sold under the brand name Tamiflu.
It was the first drug-resistant strain found in France among the 1,200 strains experts have analysed here, it said.
Speaking on Thursday, Keiji Fukuda, the WHO's special adviser on pandemic influenza, said that conclusions had still to be drawn about the reported mutations.
"The question is whether these mutations suggest that there is a fundamental change going on in viruses out there - whether there's a turn for the worse in terms of severity," he said.
"The answer right now is that we are not sure."
Dr Fukuda noted that mutations were common in influenza viruses.
"If every mutation is reported out there it would be like reporting changes in the weather," he said.
"What we're trying to do when we see reports of mutations is to identify if these mutations are leading to any kinds of changes in the clinical picture - do they cause more severe or less severe disease?
"Also we're trying to see if these viruses are increasing out there as that would suggest a change in epidemiology."
In his latest survey of swine flu developments, the WHO notes that many countries have stopped counting individual cases of swine flu, particularly of milder illness, and the case count is likely to be significantly lower than the actual number of cases that have occurred.