"We have conducted a thorough investigation to find out exactly what happened with Nico's tyre," said Pirelli motorsport head Paul Hembery.
"This investigation now excludes any structural integrity issues. Based on the information and data available an external source of damage is the conclusion made."
Pirelli said they had checked tyres used by other teams as well as Mercedes and also found nothing of any concern.
The Italian company said video footage had showed a tyre problem on the German's car "consistent with an external cut into the tyre structure".
Rosberg was fastest in Friday practise but suffered the explosive blowout in the afternoon session as he approached Blanchimont at full throttle.
"That was definitely not fun," he told reporters after escaping unscathed from the heartstopping incident in which the right rear tyre shredded in seconds.
"Luckily I did not hit the wall. I didn't feel anything before, there was no warning. There will be a big analysis. I was feeling quick before then, comfortable."
Mercedes executive director Paddy Lowe told reporters afterwards that "there seemed to be some bits of tyre structure coming out up to a minute before the actual terminal failure of the tyre."
The tyres were discussed by drivers in a meeting with FIA technical delegate Charlie Whiting on Friday evening, with reports suggesting Rosberg's world champion team mate Lewis Hamilton had raised safety concerns.
Whiting reportedly pointed out that Pirelli, whose contract runs out at the end of 2016, had not experienced any structural failures since a spate in 2013 forced changes to the tyre construction.
High-speed Spa is the longest and one of the most demanding circuits on the calendar.
(Editing by Amlan Chakraborty)
