Three men who have fathered nearly 100 children among them are doing their bit for Pakistan's skyrocketing population, which is being counted for the first time in 19 years.

But in a country where experts warn the surging populace is gouging into hard-won economic gains and social services, the three patriarchs are unconcerned. Allah, they say, will provide.

Pakistan has the highest birth rate in South Asia at around three children per woman, according to the World Bank and government figures, and the census is expected to show that growth remains high.

"God has created the entire universe and all human beings, so why should I stop the natural process of a baby's birth?" asks Gulzar Khan, a father of 36, citing one of the strongest influences in the region: the belief that Islam prevents family planning.

Tribal enmity is another factor in the northwest, where the 57-year-old lives in the city of Bannu with his third wife, who is pregnant.

"We wanted to be stronger," the 57-year-old tells said, surrounded by 23 of his offspring – so many, he observes, they don't need friends to play a full cricket match.

Polygamy is legal but rare in Pakistan, and families like Khan's are not the norm, though the beliefs he holds are widespread.

The last census, held in 1998, showed Pakistan had a population of up to 135 million. Estimates suggest the new census – carried out earlier this year, with the preliminary results due by the end of July – will put the figure closer to 200 million.

The economy is expanding faster than it has in a decade, and last month Islamabad hiked its development budget by 40 per cent.

But observers have warned the population boom is negating any progress, using up valuable resources in a young country where jobs are scarce and nearly 60 million people live below the poverty line.

"It definitely is a problem because it is affecting health outcomes adversely and also eating into development gains," says Zeba A Sathar, country director for the UN population council.

Sathar predicted the census would show growth has slowed due to lower fertility rates, though she added it would remain among the highest rates in the region.

"There has to be a clear enunciation of need to slow growth as a positive policy lever," she said. "And then next a good, effective health system that provides information and services of quality."

Khan's brother, Mastan Khan Wazir – one of his 15 siblings – also has three wives. So far, Wazir has only fathered 22 children but, like his brother, he says his grandchildren are too numerous to count.

The 70-year-old, with his bushy moustaches and bejewelled fingers, is a minor celebrity in North Waziristan tribal district, where his blinged out jeep – done in the style of Pakistan's famous "jingle trucks", with Pashto music pouring from the stereo – is a familiar sight.

"God has promised that he will provide food and resources but people have weak faith," he tells AFP, wearing a traditional Waziristani turban.

In the southwestern city of Quetta in Balochistan province, Jan Mohammed – the father of 38 children – agrees, despite having previously called on the government to provide resources for his family.

Jan said in 2016 to voice his desire for a fourth wife as he continues his mission of having 100 children. No woman has yet agreed, he says, but he has not given up.

"The more Muslims grow, the more their enemies will fear them… Muslims should go for more and more children," he said.