The Kremlin on Monday gave a stark warning against the use of U.S.-provided weapons by Ukraine to hit targets inside Russia in moves that have the potential to further escalate the ongoing eastern European military conflict.

After weeks of entreaties, Ukraine won permission to hit targets inside Russia with American-made weapons, a tactic that it says will help it defend territory in the northeast.

The recommendations apply to the northern border in Kharkiv and Sumy oblasts, where Russian forces are firing on Ukraine,” said Michael Carpenter, senior director for Europe at the U.S. National Security Council.