At the end of a regional tour, he called the withdrawal of the main Sunni bloc from the Baghdad government "discouraging."

Gates told reporters as he flew back to Washington that gains made in security in western Iraq's Anbar province and at the local level were cause for optimism, but he also acknowledged they were offset by divisions at the top.

"In some ways we probably all underestimated the depth of mistrust and how difficult it would be for these guys to come together on legislation, which let's face it is not some kind of secondary thing," he said.

"The kinds of legislation they're talking about will establish the framework of Iraq for the future, so it's almost like our constitutional convention," Gates said.

"And the difficulty in coming to grips with those we may all have underestimated six or eight months ago," he said.

All six ministers from Iraq's largest Sunni bloc tendered their resignation from Prime Minister Nuri al-Maliki's coalition government on Wednesday following a month-long spat.