The behind-the-scenes discussions, which are highly sensitive and have not been previously reported, do not put explicit geographic restrictions on the use of weapons supplied to Ukrainian forces. But the conversations have sought to reach a shared understanding of the risk of escalation, three U.S. officials and diplomatic sources said.
"We have concerns about escalation and yet still do not want to put geographic limits or tie their hands too much with the stuff we're giving them," said one of the three U.S. officials, speaking on condition of anonymity.
President Joe Biden's administration and U.S. allies have been increasingly willing to give Ukraine longer-range weaponry, including M777 howitzers, as Kyiv battles Russian forces with more success than American intelligence officials had predicted. The Pentagon's announcement last week that Denmark will provide Ukraine with Harpoon anti-ship missiles would further extend Kyiv's reach.
After initially forecasting Ukraine would be overrun by Russia's much bigger military, American officials have recently voiced hopes that Ukrainian forces can win the war, and want to arm them to do so.
U.S. officials say the Biden administration is even considering supplying Kyiv with the M142 High Mobility Artillery Rocket System (HIMARS), which depending on the munitions can have a range of hundreds of kilometers.
But U.S. intelligence has also warned about growing risks, particularly given a mismatch between Russian President Vladimir Putin's apparent ambitions and the performance of his military. The coming months could put the war on a "more unpredictable and potentially escalatory trajectory," Director of National Intelligence Avril Haines told a Senate hearing this month.
The United States, by design, is not directly combating Russian forces but Pentagon commanders are in constant contact with Ukrainian leaders and have provided critical intelligence that has allowed Ukraine to target Russian troops, on land and at sea, U.S. officials have said.
A second U.S. official, also speaking on condition of anonymity, said Washington and Kyiv had a shared "understanding" about the use of certain Western-provided weapon systems.
"So far, we've been on the same page about the thresholds," the official said.
Ukraine's defence ministry did not immediately respond to a request for comment.
Russian Foreign Minister Sergei Lavrov warned the West that supplying weapons to Ukraine capable of hitting Russian territory would be "a serious step towards unacceptable escalation," according to remarks published on the Russian foreign ministry website on Thursday.
