Mobile internet access in Ethiopia has been blocked by the government - but officials are refusing to say why.
It was reported last year that the country banned access to social media sites after university entrance exams were leaked online.
There has been speculation that similar motives lie behind the latest "blackout", although some reject this.
A state of emergency has been in effect in Ethiopia since October, following anti-government protests.
Mobile internet access has been disabled on other occasions recently, including a period of a few weeks last year.
"Mobile data has been deactivated," deputy communications minister Zadig Abraha told the AFP news agency on Wednesday.
On Thursday, the BBC was unable to access websites belonging to the Ethiopian government and the sole communications provider, Ethio Telecom.
There were reports that fixed-line broadband connections had also been affected.
Preliminary data from Google suggested that there had indeed been a big drop in Ethiopian internet traffic to Google services since Wednesday afternoon.