WASHINGTON (AP) — President Barack Obama said Wednesday that U.S. sanctions levied against Russia over its actions in Ukraine are working but that Washington would face a much different set of questions about how to respond if Moscow invaded eastern Ukraine.

Western officials warn that a Russian military buildup on Ukraine’s border could herald a major incursion to protect the pro-Moscow separatists fighting Ukrainian forces. Despite bipartisan pressure from lawmakers to send U.S. military aid to Ukraine, however, the Obama administration thus far has said it doesn’t believe U.S. military assistance is needed.