ABOARD AIR FORCE ONE, Aug 20 (Reuters) - U.S. Ambassador to Iraq Ryan Crocker and the top U.S. commander in Iraq, Gen. David Petraeus, will likely testify to Congress about progress in the war on Sept. 11 or Sept. 12, the White House said on Monday. White House spokesman Gordon Johndroe, speaking on Air Force One headed with President George W. Bush to a summit in Canada, said the hearing date was not related to the anniversary of the 2001 attacks. The September progress report is seen in Washington as a pivotal milestone in assessing whether the Pentagon's so-called surge strategy has worked. That strategy has aimed to establish enough security to allow Iraqi politicians to move toward reconciliation. The report was due on Sept. 15, a Saturday. Johndroe said congressional schedules would likely dictate a Sept. 11 or Sept. 12 hearing date. (Additional reporting by Kristin Roberts)