Image copyright AP Image caption The authorities have recently begun a new polio vaccination campaign

Three polio workers have been killed in the southern Pakistani city of Karachi, a day after authorities began a new vaccination drive, officials say.

Reports say gunmen opened fire in the Qayumabad area, killing one man and two women administering polio drops.

The attack is the latest in a series targeting polio teams in the country.

No group has claimed responsibility, but the Taliban oppose the polio schemes, which they see as a cover for international espionage.

Pakistan is one of only three countries where polio remains endemic and is indeed on the rise, due in part to this militant resistance to polio mass vaccination campaigns.

Nigeria and Afghanistan are the others, but WHO officials have stressed that progress is being made in these countries.

Earlier this month India declared itself polio-free, three years after its last reported case - a landmark in the global battle against the disease, and largely due to a massive and sustained vaccination campaign.

But Pakistan has witnessed a campaign of violence against health workers, who militants also accuse of being part of a Western plot to sterilise Muslims.

In December three people working on polio vaccination teams were shot dead in north-west Pakistan, even though the vaccination scheme won the backing of a prominent religious seminary said to wield influence over Taliban groups.

The latest attack comes one day after health authorities in Sindh province, of which Karachi is the capital, began to inoculate 7.6m children as part of a nationwide. vaccination campaign.