The U.S. government on Wednesday warned that a hacking group widely known as cloudhopper, which Western cybersecurity firms have linked to the Chinese government, has launched attacks on technology service providers in a campaign to steal data from their clients.

The Department of Homeland Security issued a technical alert for cloudhopper, which it said was engaged in cyberespionage and theft of intellectual property, after experts with two prominent U.S. cybersecurity companies warned earlier this week that Chinese hacking activity has surged amid the escalating trade war between Washington and Beijing.

Chinese authorities have repeatedly denied claims by Western cybersecurity firms that it supports hacking.

Homeland Security released the information to support U.S. companies in responding to attacks by the group, which is targeting information technology, energy, health care, communications and manufacturing firms.

“These cyber threat actors are still active and we strongly encourage our partners in government and industry to work together to defend against this threat,” DHS official Christopher Krebs said in a statement.

The reported increase in Chinese hacking follows what cybersecurity firms have described as a lull in such attacks prompted by a 2015 agreement between Chinese President Xi Jinping and former U.S. President Barrack Obama to curb cyber-enabled economic theft.

“I can tell you now unfortunately the Chinese are back,” Dmitri Alperovitch, chief technology officer of U.S. cybersecurity firm CrowdStrike, said Tuesday at a security conference in Washington, D.C.

“We’ve seen a huge pickup in activity over the past year and a half. Nowadays they are the most predominant threat actors we see threatening institutions all over this country and Western Europe,” he said.

Analysts with FireEye, another U.S. cybersecurity firm, said that some of the Chinese hacking groups it tracks have become more active in recent months.

Wednesday’s alert provided advice on how U.S. firms can prevent, identify and remediate attacks by cloudhopper, which is also known as Red Leaves and APT10.

The hacking group has largely targeted firms known as managed service providers, which supply telecommunications, technology and other services to business around the globe.

Managed service providers, or MSPs, are attractive targets because their networks provide routes for hackers to access sensitive systems of their many clients, said Ben Read, a senior intelligence manager with FireEye.

“We’ve seen this group route malware through an MSP network to other targets,” Read said.