Saudi Aramco said Thursday that company data had been leaked from one its contractors, but said it had "no impact" on the oil giant's operations.

Aramco, the kingdom's cash cow and the world's biggest energy company, did not respond to questions about a possible cyber attack or any ransom demand.

"Aramco recently became aware of the indirect release of a limited amount of company data which was held by third party contractors," the oil giant said in a statement.

"We confirm that the release of data was not due to a breach of our systems, has no impact on our operations, and the company continues to maintain a robust cyber security posture."

So called "ransomware" attacks are an increasingly lucrative form of digital hostage-taking, in which hackers encrypt victims' data and then demand money for restored access.

Saudi Arabia, the world's largest exporter of crude oil, has been criticised for the vulnerability of its infrastructure, and has been the target of cyber attacks in the past.

In 2012, Aramco was among the firms hit by Shamoon, an aggressive disc-wiping malware, in what is believed to be the country's worst cyber attack yet.

At that time, US intelligence officials suspected the involvement of Iran, Saudi Arabia's regional rival.

In 2016, Saudi media reported that cyber attacks had targeted several state institutions, as well as other vital structures.