The Afghan administration offered no immediate explanation for the outage and could not be reached for comment.
The United Nations called on authorities to fully restore connections.
In the past, the Taliban have voiced concern about online pornography, and authorities cut fibre-optic links to some provinces in recent weeks, with officials citing morality concerns.
"The cut in access has left Afghanistan almost completely cut off from the outside world, and risks inflicting significant harm on the Afghan people," the UN said in a statement.
Afghanistan is grappling with the aftermath of an earthquake that hit the east of the country, the return of millions of refugees expelled from neighbouring countries and a drought in the north.
An Afghan mobile phone services provider, which did not want to be named, said it was working alongside other providers "to manage this sensitive and complex situation".
"We are following directives from the authorities and we hope that all telcos in the country will be authorised to recommence services as soon as possible," the company said.
Internet connectivity in Afghanistan was flatlining about the one per cent mark, international internet access monitoring organisation NetBlocks said.
Connectivity was cut in phases on Monday, with the final stage affecting telephone services, which share infrastructure with the internet, NetBlocks said in an email to Reuters.
Private channel Tolo News, which warned viewers of a disruption to its services, said authorities had set a one-week deadline for the shutdown of 3G and 4G internet services for mobile phones, leaving only the older 2G standard active.
The channel also reported disruption for private banks and the central bank while Kabul's currency market was working off yesterday's exchange rates.
Shabeer, who gave only his first name, said that his job at a private internet service provider had been suspended.
"People today rely on technology, it is the main way to stay connected with the outside world," he said.
"No one knows about the condition of their relatives, and even within Afghanistan, we cannot communicate."
Arafat Jamal, country representative for the UN refugee agency, told reporters via satellite link from Kabul that it could no longer reach its frontline aid workers, including those helping with the earthquake response.
"It is another crisis on top of the existing crisis. It is utterly unnecessary for this kind of interruption to take place, and the impact is going to be on the lives of Afghan people," he said, adding that it was seeking a waiver from the blackout from authorities.
