By Phil Stewart and Idrees Ali WASHINGTON (Reuters) – As the United States and its allies provide Ukraine with increasingly sophisticated arms, Washington has held discussions with Kyiv about the danger of escalation if it strikes deep inside Russia, U.S. and diplomatic officials tell Reuters. 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. offi…