Florida ? Any effort to legislate a deadline for U.S. withdrawal from Iraq will “make it difficult if not impossible” for military commanders to do their mission, Defense Secretary Robert Gates said Friday.

Both the Pentagon and the Iraqi government have gotten the message that Congress is losing patience with the war, said Gates, speaking to reporters as he flew back to Washington from a ceremony installing the new U.S. commander for the Middle East.

It’s clear, he said, “that there’s a great deal of impatience to see progress. Congress doesn’t want to see an open-ended conflict,” Gates said, “If the intention was to send a message, that message has been received – not just by us …but by the Iraqis.”

Gates flew to Tampa for a change-of-command ceremony where Navy Adm. William Fallon was installed as the new head of U.S. Central Command.

In remarks at the ceremony, Gates said the U.S. is ready to defend its interests in the Middle East for decades to come, even though the Iraq war has been tougher than expected.