Pakistan's military said its forces killed more than 50 Afghan soldiers and destroyed five checkpoints in heavy fighting along their disputed border, a claim quickly rejected by Kabul.

The clashes took place on Friday at the Chaman border that divides Pakistan's southwest Balochistan province and Afghanistan's southern Kandahar, as Pakistani officials were carrying out a census count.

At least eight civilians were killed, according to previously stated death tolls by officials - seven on the Pakistani side and one Afghan.

On Sunday, Pakistan elevated its rhetoric by saying Afghan forces had suffered much more dramatic losses.

READ MORE: Pakistan-Afghanistan crossing closed after border clash

"We are not pleased to tell you that five Afghan check posts were completely destroyed - more than 50 of their soldiers were killed and above 100 were wounded," Major-General Nadim Ahmad, head of the paramilitary Frontier Corps, said.

"We are not happy for their losses, but we were forced to retaliate."

Ahmad said two Pakistani soldiers were killed and nine more wounded in the incident.

Afghanistan quickly denied the statement.

"Very false claims by Pakistani Frontier Corp that as many as 50 Afghan soldiers lost their lives in Pak retaliation; totally rejected," Sediq Sediqqi, a government spokesman, said on Twitter.

Al Jazeera's Qais Azimy, reporting from Kabul, said Afghan officials called Pakistan's claims "totally false".

"Afghan security official at the border confirmed to us at least four policemen killed, and that one woman, a civilian, was killed as a result of artillery attack by Pakistani forces," he said.

He said Afghan officials also denied the claim that Pakistani civilians were killed.

The clashes prompted thousands of families to flee the area, he said.

"Residents are worried that fighting could start any minute because security forces remain in the area. That's why they are leaving."

According to Pakistan, the fighting began when Afghan troops fired on Pakistani census workers.

They said the Afghan government had been notified and given the coordinates of the border villages, where the census workers were going door to door.

Afghan officials, however, said Pakistani troops fired the first shots.

They blamed Pakistani census enumerators, who were accompanied by soldiers, for straying across the border, a charge Islamabad denied.

The so-called Durand Line, a 2,600km frontier drawn by the British in 1896 and disputed by Afghanistan, has witnessed increased tension since Pakistan began patrolling along it last year.

The border has remained closed since Friday, with senior Pakistan army general Amir Riaz telling reporters it would remain so "until Afghanistan changes its behaviour".

It is not the only area of dispute between the neighbours: They accuse each other or harbouring armed groups who carry out attacks across their borders.

Pakistan embarked on the enormous task of conducting its first census in almost two decades in March.

Pakistan is the sixth most populous in the world with an estimated 200 million people, but has not held a census since 1998, despite a constitutional requirement for one every decade.