GENEVA--Swiss drug maker Novartis on Tuesday said it has received samples of the swine flu A(H1N1) virus and is awaiting the World Health Organization’s go-ahead to begin producing a vaccine.

"We have received the virus and our researchers have started looking into modifying it so we can begin producing a vaccine," said a spokesman who added that it would take three to four weeks to get up and running.

"We are waiting for the green light from the WHO and the CDC (the US Centers for Disease Control and Prevention)," the spokesman said.

Novartis already produces a vaccine against seasonal flu but now has to work out what priority to give to production demands for each vaccine given potentially different needs for the northern and southern hemispheres, he added.