American officials said the tone of the note was consistent with a series of public Russian threats, including to target deliveries of weapons as they moved across Ukrainian territory.
Officials said the note did not prompt any special concern inside the White House. But it has touched off a broader discussion inside the Pentagon and intelligence agencies about whether the “unpredictable consequences” could include trying to target or sabotage some of the weapons shipments while still in NATO territory, before they are transferred to Ukrainians for the final journey into the hands of Ukrainian troops. The delivery of the protest note was first reported by The Washington Post.
The weapons President Biden authorized this week for transfer to the Ukrainians include long-range artillery that is suited for what American officials believe will be a different style of battle in the open areas of the Donbas, where Russian forces appear to be massing for an attack in coming days.
Pentagon officials were insistent in the run-up to the war that the United States provide only defensive weaponry that would avoid escalation.
Jake Sullivan, the president’s national security adviser, described in an interview at the Washington Economic Club on Thursday how he and Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, had reviewed weapons requests. They went over each item with their Ukrainian counterparts, talking about what the United States had in its stocks and what it could deliver quickly.
Reports by pro-Kremlin media have highlighted antitank systems and other Western weapons used by Ukrainian forces, promoting the idea that Russia is not at war with Ukraine but with an American-led alliance seeking to destroy Russia. Mr. Biden and his aides have denied that, saying that they wished to avoid direct conflict with Russia and had no interest in American-engineered regime change.