The United States believes that the Ukrainians were behind the assassination in Russia

He said: “We have other targets on the territory of Ukraine, I mean collaborators and representatives of the Russian leadership, who may be of value to members of our special services working in this program, but certainly not Dugina.”

Although the details surrounding the sabotage in Russian-controlled territory are shrouded in mystery, the Ukrainian government has quietly acknowledged the killing of Russian officials in Ukraine and the sabotage of Russian arms factories and arms depots.

A senior Ukrainian military official, who asked not to be named due to the sensitivity of the topic, said that Ukrainian forces, with the help of local fighters, carried out assassinations and attacks on accused Ukrainian collaborators and Russian officials in the occupied Ukrainian territories. Among them is the head of the Kherson region who was installed by the Kremlin, poisoned in August and had to be transferred to Moscow for emergency treatment.

Countries traditionally do not discuss other countries’ covert actions, for fear of revealing their own operations, but some US officials believe it is important to rein in what they see as dangerous adventures, particularly political assassinations.

However, US officials have made an effort in recent days to insist that relations between the two governments remain strong. US concerns about aggressive covert operations inside Russia have not led to any known changes in the provision of intelligence, military, and diplomatic support to Mr. Zelensky’s government or to Ukraine’s security services.

In a phone call on Saturday, Secretary of State Anthony J. Blinkin told his Ukrainian counterpart Dmytro Kuleba, that the Biden administration “will continue to support Ukraine’s efforts to regain control of its territory by strengthening its hand militarily and diplomatically,” according to Ned Price, a State Department spokesman.

Officials from the State Department, the National Security Council, the Pentagon and the CIA declined to comment on the intelligence assessment.