Pakistan authorities blamed a mystery months-long internet slowdown that has drawn backlash from activists and business leaders on damaged underwater cables.

Digital rights experts believe the state is testing a firewall -- a security system that monitors network traffic but can also be used to control online spaces.

The government has previously blamed a surge in VPN use for the slowdown whilst also admitting that the country was "undergoing a transition".

"The ongoing internet slowdown across the country is mainly due to (a) fault in two of the seven international submarine cables connecting Pakistan internationally," Pakistan's Telecommunications Authority said in a statement on Wednesday, adding that the fault would be repaired by early October.

Internet networks have been up to 40 percent slower than normal since July, according to one IT association, while WhatsApp and VPN connections are severely disrupted.

The government and PTA for weeks refused to comment on the slowdown.

At the start of the month, Defence Minister Khawaja Asif said the country was "undergoing a transition."

He added that "there will be some controls to prevent threatening and defamatory content against the state and individuals."

IT minister Shaza Fatima Khawaja later denied that the government was behind the internet slowdown, blaming it on a surge in VPN use.

It comes as Pakistan's military -- the country's most powerful institution -- says it is battling so-called "digital terrorism".

Analysts say the main target of the digital disruption is the party of jailed opposition leader Imran Khan, still wildly popular and boosted by a young, tech-savvy voter base.

Global rights watchdog Amnesty International urged Pakistan authorities to be transparent. "The opacity of the Pakistani authorities regarding the use of monitoring and surveillance technologies that block content, slow down and control internet speeds is an alarming concern," said the organisation's technology expert Jurre Van Bergen.

Pakistan is banking on its nascent but growing Information Technology industry to increase its exports and generate critical foreign exchange revenue for a cash-strapped country.

"Without immediate and decisive action, the country risks deeper economic fallout and a prolonged digital divide," Shahzad Arshad, the chairman of the Wireless Internet Service Providers Association of Pakistan, said in a statement.