The World Health Organization is looking for more information on reports of a drug-resistant strain of swine flu in Britain and a batch of H1N1 vaccine that was put on hold in Canada following allergic reactions.

The UN health agency is also checking whether a mutation in the H1N1 strain reported by the Norwegian Institute of Public Health last Friday causes more serious illness by allowing the virus to go deeper into the respiratory system.

The agency is gathering information from Canadian officials about a batch of 172,000 doses of H1N1 vaccine that may have caused more allergic reactions than normal.

"An unusual number of severe allergies to the vaccine have been detected in Canada," WHO spokesman Thomas Abraham told Agence France-Presse on Tuesday.

Vaccine maker GlaxoSmithKline asked provinces last week to stop administering Aprepanrix vaccine from the affected lot, linked to at least six severe reactions. The company has not said how many doses had been used. Those who suffered reactions have recovered.

Abraham also said that the reported Norwegian mutation "is a major issue we are looking at."

"If the mutation in fact is associated with severe cases then we really need to know about it," he told Reuters in Geneva. "This might be a signal. We need to investigate," he added.

"As of now there is no evidence of a particular association with severe cases."

A similar mutation has been reported in Brazil, China, Japan, Mexico, Ukraine and the United States, WHO said.

Also last Friday, Britain's Health Protection Agency said five cases at a hospital in Wales were known to be resistant to antiviral drug Tamiflu, also known as oseltamivir.