Microsoft has become the latest US technology company to confirm that it has been targeted by computer hackers.
In a blog post, Microsoft announced that “a small number” of its computers had recently been deliberately infected with malicious software.
The firm said it found no evidence that any customer data had been accessed, but an investigation is continuing.
On Tuesday Apple said its computers were attacked by the same hackers who targeted Facebook a week earlier.
At the time, Facebook said it had traced a cyber attack back to China which had infiltrated employees’ laptops.
In Friday’s blog post, Microsoft spokesman Matt Thomlinson said: “This type of cyberattack is no surprise to Microsoft and other companies that must grapple with determined and persistent adversaries.”