U.S. Sen. Jim Webb, D-Va., chairman of the Personnel Subcommittee of the Senate Committee on Armed Services, said U.S. combat forces are stressed to the limit following years of continuous war in Iraq and Afghanistan.
"We are entering uncharted territory in terms of the long-term consequences of past rotation cycles and an unsatisfactory deployment-to-dwell ratio," he said in a statement.
Monday marks the final day before U.S. forces in Iraq change missions from a combat role under Operation Iraqi Freedom to an advisory role under Operation New Dawn.
The last combat brigade left Iraq Aug. 19. There are fewer than 50,000 U.S. troops in Iraq for the first time since the March 2003 invasion.
Violence in Iraq soared in the last week. Bombings killed at least 50 people Wednesday and injured hundreds more across 13 cities in northern, western and southern Iraq.
U.S. President Barack Obama is to deliver a prime-time address Tuesday to mark the change in the mission.
All of the U.S. troops stationed in Iraq are to leave the country by the end of next year.
Webb's comments were to highlight a September meeting to discuss the psychological and physical trauma of disabled war veterans.