Chinese government-backed hackers penetrated several U.S. Treasury Department workstations and obtained unclassified documents, according to a letter sent by the department to lawmakers on Monday.

Treasury officials wrote that the hack was carried out by a “China state-sponsored Advanced Persistent Threat (APT) actor” who gained access by compromising a private software service vendor. The vendor, Atlanta-based BeyondTrust, alerted the government on Dec. 8 that the hackers had “gained access to a key used by the vendor to secure a cloud-based service used to remotely provide technical support for Treasury Departmental Offices.”

Treasury officials did not reveal how many workstations were accessed or what sort of documents the hackers may have obtained, but said there is so far “no evidence indicating the threat actor has continued access to Treasury information.” At the same time, the department said the hack was being investigated as a “major cybersecurity incident.”

China’s Foreign Ministry in a briefing in Beijing Tuesday called the latest U.S. accusations “groundless,” its standard response to hacking allegations.

The development comes as U.S. officials grapple with the fallout of a massive Chinese cyberespionage campaign known as “Salt Typhoon” that gave officials in Beijing access to private texts and phone conversations of an unknown number of Americans. According to The Associated Press, a top White House official said Friday that the number of telecommunications companies affected by the Salt Typhoon hack has now risen to nine.

The Pentagon and the Chinese government are both keeping quiet for now on recent video images of two new fighters spotted on Chinese social media sites that defense analysts say appear to be Beijing’s first advanced sixth-generation stealth fighters.

An Air Force official tells National Security Correspondent Bill Gertz that the service is closely watching Beijing’s advanced warplane development. The Air Force views the two prototypes revealed last week as in line with Chinese strategic objectives and long-range force planning, the official said.

At the Pentagon, deputy press secretary Sabrina Singh told reporters on Monday she had seen the news reports but declined to comment on the flight tests. A second Pentagon spokesman said the Defense Department had no comment on China’s development of new warplanes beyond the recently published annual report on the Chinese military.

A Chinese Embassy spokesman also declined to comment. Chinese state media carried no stories directly commenting on the two jets. The Defense and Foreign Ministries in Beijing so far remained silent on the online appearance of the jets, which were widely circulated globally on the internet.

The Defense Department is again dipping into its own weapon stores to provide Ukraine with another tranche of military aid in the waning weeks of the Biden administration. U.S. officials on Monday announced more than $2 billion in fresh air defense weapons, artillery rounds and attack drones for Kyiv, with roughly half to be drawn from existing U.S. military supplies.

Administration officials announced a $1.25 billion package — the 73rd time the Pentagon used the Presidential Drawdown Authority (PDA) to supply Kyiv with military assistance since Russia launched its invasion in February 2022. A separate $1.22 billion Ukraine Security Assistance Initiative was also announced. Unlike a PDA, it won’t be drawn from existing U.S. stocks, but will go toward delivering longer-term weapons packages.

Mr. Trump and some of his closest advisers have expressed skepticism of the tens of billions of dollars of aid given to Ukraine since the war started, and the Biden administration has been approving new shipments at a vigorous rate ahead of Mr. Trump’s Jan. 20 inauguration. The aid announced Monday includes munitions for the U.S.-supplied National Advanced Surface-to-Air Missile Systems (NASAMS), HAWK air defense munitions, ammo for High Mobility Artillery Rocket Systems (HIMARS), and 155mm and 105mm artillery rounds.

Pamela K. Isom homes in on the recent “Salt Typhoon” cyberattacks in which hackers affiliated with China’s Ministry of State Security allegedly compromised several telecommunications companies, extracting lawful wiretaps and critical intelligence data from some of the companies.

Writing in an op-ed published prior to the latest revelations that Chinese hackers have separately penetrated several U.S. Treasury Department workstations, Ms. Isom asserts that the Salt Typhoon attacks revealed “vulnerabilities across the telecommunications sector” and highlighted “the urgent need for systemic change in how telecommunications providers approach cybersecurity.”

“Government and industry leaders must be proactive and prioritize threat intelligence to improve defenses before incidents occur. This requires proactive threat intelligence-sharing,” writes Ms. Isom, the CEO and founder of IsAdvice & Consulting and a member of the American Security Project’s Consensus for American Security. “Encouraging an exchange between federal agencies and industry peers on actionable intelligence about emerging threats before incidents occur would help secure our country against state-sponsored hackers.”

Former Rep. Devin Nunes, Mr. Trump’s designated intelligence adviser and slayer of the discredited anti-Trump Steele dossier, tells Washington Times opinion columnist Rowan Scarborough that he wants the “infected-by-politics” U.S. intelligence community to clean up its partisan “politicization” and that Mr. Trump is “determined to root it out.”

Mr. Trump has named Mr. Nunes, a California Republican and former House Intelligence Committee chairman, to chair the President’s Intelligence Advisory Board, where he will have direct access to the president on all matters of intelligence, writes Mr. Scarborough.

Mr. Trump will hear other voices, including those of the director of national intelligence, slated to be former Democratic Rep. Tulsi Gabbard, and the nominee for CIA director, John Ratcliffe. “But it is Mr. Nunes with whom Mr. Trump has forged a strong alliance,” writes Mr. Scarborough. “In 2021, he hired him as chairman and CEO of his new Trump Media & Technology Group, which includes the Trump-dominated blogging site Truth Social.”

• Jan. 6 — NVIDIA CEO Keynote at CES, NVIDIA

• Jan. 7 — Report Launch and Panel: NATO and U.S.-Turkey Defense Cooperation in a New Era, Atlantic Council

• Jan. 7 — The Hidden Costs: Transparency and the U.S. Arms Trade, Stimson Center

• Jan. 7-10— CES 2025, Consumer Technology Association

• Jan. 8 — Russia’s Difficult Road to Freedom: A Conversation with Vladimir Kara-Murza, American Enterprise Institute

• Jan. 8 — The Role of Religion and Spirituality in U.S. Security Assistance, Center for Strategic and International Studies

• Jan. 9 — What Do People in Taiwan and the United States Think about Taiwan’s Security Situation? Brookings Institution