The death toll in a swine flu outbreak in India since the beginning of this year has risen to 21, officials say.

Most of the deaths have been reported from the western state of Maharashtra. Rajasthan, Andhra Pradesh and Karnataka have also reported deaths.

Over 300 others have been infected with the virus, and many of them admitted to hospitals.

The cause of the sudden outbreak is not clear. The virus killed more than 450 Indians when it first emerged in 2009.

Health officials say the rise in new cases could be a "short spurt".

"There is no need to panic. People need to exercise precautionary measures to ward off the infection," health ministry official Avdesh Kumar was quoted as saying by The Times of India newspaper.

The western Indian city of Pune is the worst hit. Most of the deaths have happened since March.

More than 450 people died of swine flu in India when it first broke out in 2009. More than 13,000 people were infected with the virus.

The states with the highest number of swine flu deaths in 2009 included Maharashtra, Karnataka and Gujarat.

The virus is thought to have killed more than 1,200 people around the world.

The swine flu (H1N1) virus first emerged in Mexico in April 2009 and has since spread to many countries.