Gunmen on motorcycles gunned down seven policemen guarding a polio vaccination team in Pakistan's southern city of Karachi on Wednesday, officials said.

Feroz Shah, a senior police official told AFP that eight gunmen carried out the killings in two separate attacks in the city's western Orangi Town neighbourhood.

"The gunmen first opened fire on three policemen in the streets of Orangi Town, killing them all," he said, adding: "Later they shot dead four policemen, who were sitting in a police mobile van" a few streets away.

Abdul Kareem, an official in Abbasi Shaheed Hospital where the bodies were taken, also confirmed the casualties.

Pakistan is one of only two countries where polio, a crippling childhood disease, remains endemic. Attempts to eradicate it have been badly hit by militant attacks on immunisation teams that have claimed nearly 80 lives since December 2012.

Islamist groups including the Taliban say the polio vaccination drive is a front for espionage or a conspiracy to sterilise Muslims. 

In 2014 the number of polio cases recorded in Pakistan soared to 306, the highest in 14 years, before falling to 52 in 2015.

The most recent attack came in January, when a suicide bomber blew himself up outside a polio vaccination centre in the southwestern city of Quetta, killing 15 people -- two civilians and 13 security officials.