MOSUL, Iraq — Defense Secretary Robert Gates is using his latest unannounced trip to Iraq to assess whether the downturn in violence in parts of the country can be sustained and whether Iran is working to quell the shipment of arms into Iraq, officials said today as the Pentagon chief made his sixth visit to the country.

Senior Defense officials said the jury is still out on both fronts, and the Pentagon is being cautious not to declare victory yet in either case.

As an example, one senior defense official traveling with the secretary said that any proposal to move the Marines out of Iraq and into a greater role in Afghanistan, as proposed not long ago by Marine Commandant Gen. James Conway, will be viewed cautiously by Gates.

Gates arrived in Mosul at midmorning today and planned a full day of meetings with his military commanders and Iraqi officials.

Army Col. Tony Thomas, a brigade commander, told reporters traveling with Gates that he and other senior commanders in the north are looking for additional U.S. troops and also would like the return of 1,400 Iraqi troops sent to Baghdad to provide "more combat power" to help stabilize a number of areas — including Diyala province, Mosul and Samara — to help counter an uptick in violence, including suicide bombings.

Overall, there has been a steady decline in violence in Iraq in recent months, including dips in roadside bombs, other attacks and in overall casualties. The U.S. is pressing Iraqi leaders to take advantage of the improved security to make the political reforms needed to stabilize the fledgling democracy.

Defense officials also said they are still searching for answers on how to sustain the lower violence levels because the cause of the increased security has varied according to the region.