US convenience store chain, 7-Eleven, said Monday that it had closed its outlets in Denmark after a suspected hacker attack knocked out their cash tills.

"We suspect that we have been the victim of a hacker attack today, Monday, August 8", the group which runs 175 convenience stores in Denmark wrote on Facebook.

"We can't use the cash registers nor accept payments. We are therefore closed until we know the extent" of the hacker attack, the message read.

The Danish manager for the retailer said the incident occurred suddenly on Monday.

"The cash registers just stopped working in all the stores and the employees started letting us know. This has never happened before", Jesper Ostergaard told Danish public television DR.

It was not known when the stores would reopen.

"There is no schedule yet", Ostergaard said.

Based in Irving, Texas, 7-Eleven has than 77,000 stores worldwide which it operates either directly or under licence.