FARGO, N.D. – Senator Barack Obama said Thursday the United States cannot sustain a long-term military presence in Iraq, but added that he would be open to “refine my policies” about a timeline for withdrawing troops after meeting with American military commanders during a trip to Iraq later this month.

Mr. Obama, whose popularity in the Democratic primary was built upon a sharp opposition to the war and an often-touted 16-month gradual timetable for removing combat troops, dismissed suggestions that he was changing positions in the wake of reductions in violence in Iraq and a general election fight with Senator John McCain.

“I’ve always said that the pace of withdrawal would be dictated by the safety and security of our troops and the need to maintain stability. That assessment has not changed,” he said. “And when I go to Iraq and have a chance to talk to some of the commanders on the ground, I’m sure I’ll have more information and will continue to refine my policies.”

As he arrived for a campaign stop in North Dakota, Mr. Obama told reporters on Thursday that he intended to conduct “a thorough assessment” of his Iraq policy during a forthcoming trip to the country. He stressed that he has long called for a careful and responsible withdrawal of American forces, but he declined to offer a fresh endorsement of his plan to remove one to two combat brigades a month.

“My 16-month timeline, if you examine everything that I’ve said, was always premised on making sure that our troops were safe,” he said. “I said that based on the information that we had received from our commanders that one to two brigades a month could be pulled out safely, from a logistical perspective. My guiding approach continues to be that we’ve got to make sure that our troops are safe and that Iraq is stable.”

He added, “I’m going to continue to gather information to find out whether those conditions still hold.”

It’s been more than two years since Mr. Obama has visited Iraq, which Republicans have used as a point of criticism. After dismissing an invitation from Mr. McCain to visit Iraq together this summer as a “political stunt,” Mr. Obama began making preparations for his own trip to Iraq.

Dates of his visit have not been disclosed for security reasons, aides said, but his trip will be part of a Congressional delegation in his capacity as a member of the Senate Foreign Relations Committee.

“My job is to make sure that the strategic issues that we face, not just in Iraq, but in Afghanistan, Iran and Pakistan,” Mr. Obama said, “that those are all taken into account and dealt with in a way that enhances America’s national security interests over the long term.”