North Korea’s cyber soldiers have deployed targeted attacks against South Korea’s defense industry in recent days, security researchers say, in a campaign timed to coincide with the start of the first large-scale U.S.-ROK military exercises in years.

Cyber firm EST Security announced the suspected DPRK-backed attacks Thursday in a blog post, saying hackers had been caught distributing malicious text documents and tailor-made phishing sites since Aug. 22. 

The attacks align with the start of Ulchi Freedom Shield (UFS), which kicked off on Monday and drew immediate condemnation from Pyongyang, including threats of