Officials initially resisted such a move, citing escalation concerns and uncertainty over how Putin would respond. Some of those officials, including in the White House, the Pentagon and the State Department, feared lethal retaliation on U.S. military and diplomatic personnel and attacks on NATO allies.

President Vladimir Putin said on Thursday that the Ukraine war was escalating towards a global conflict after the United States and Britain allowed Ukraine to hit Russia with their weapons, and warned the West that Moscow could strike back.

U.S. President-elect Donald Trump has famously promised to end Russia’s war in Ukraine on his first day in office, but he has been characteristically vague on how he will do it.