Numsa has said a break-in in its Johannesburg offices - in which "highly sensitive" computer equipment was stolen - was "well-orchestrated".
The National Union of Metalworkers South Africa (Numsa) has reported what it calls a "politically motivated" burglary to the police.
Numsa spokesperson Castro Ngobese said unknown men broke into the union's Johannesburg offices on Thursday night, stealing "highly sensitive" computer equipment.
The incident was reported to the police on Friday, although the union said it had received no feedback from the police about prior, similar incidents.
According to Ngobese, Numsa believes the incident to be a tactic aimed at intimidating its leadership. The union has aligned itself with suspended Cosatu general secretary Zwelinizima Vavi.
Earlier this month, Vavi asked the high court in Johannesburg to overturn his suspension.
Said Ngobese: "We are adamant that this latest burglary or break-in is not an act of criminality, but is a well-targeted and well-orchestrated work of highly sophisticated and trained individuals.
"We are suspicious that the burglary or break-in is politically motivated, and is intended to intimidate and threaten Numsa and its elected leadership. Everybody knows that Numsa has become a political target not only from our class opponents, but also from those we regard as our comrades and brothers-in-arms," he said.
Numsa said it wouldn't be surprised if it was now accused of being paranoid.
"We have never thought that in a democratic state we must deal with issues of security of our properties, in the similar way we did during apartheid. Our offices are a collective property of workers and we will never auction them to the enemy camp or class collaborators," Ngobese said.
He added that the union would take "extraordinary measures" to secure its property.