Woolworths has increased vigilance at its stores nationwide after improvised explosive devices were detonated at its Menlyn Park and Preller Square branches.

An explosives expert has warned that two improvised explosive devices detonated at Woolworths stores in Pretoria and Bloemfontein could have been “possibly lethal,” even though no injuries were reported.

Willem Els, a former SAPS explosives investigator and consultant with the Institute for Security Studies, indicated that the incidents may be linked to an extortion campaign against the retailer.

"A bomb is a bomb. It will always be dangerous even if no one was hurt," Els said.

The first explosion took place in the early hours of Thursday, May 28, at Woolworths Menlyn Park in Pretoria. The second happened less than 24 hours later, in the early hours of Friday, May 29, at the Woolworths Preller Square branch in Bloemfontein.