Washington: Senator Barack Obama said on Thursday that he might "refine" his Iraq policies after meeting with military commanders there later this summer. But hours later he held a second news conference: to emphasise his commitment to the withdrawing of all combat troops from Iraq within 16 months of taking office.

His two statements in Fargo, North Dakota, reflected how the changing dynamics in Iraq have posed a challenge for Obama, the presumptive Democratic presidential nominee. He has been trying to retain flexibility as violence declines there without abandoning one of the central promises of his campaign: that if elected he would end the war.

His remarks came as Republicans - including his all-but-certain Republican rival, Senator John McCain - have been arguing that Obama would most likely change his position on the phased withdrawal. They argue that with violence dropping there, bringing the troops home would risk erasing the fragile gains that have been made.

Obama said at his first news conference that he planned a "thorough assessment" of his Iraq policy when he visits the country later this summer. "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," he said. "That assessment has not changed. 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."

Obama's shift in emphasis in the way he spoke about the situation on Thursday fueled speculation that he might not be wedded to his timetable.

So the Obama campaign scheduled a second news conference to try to clarify his remarks. "We're going to try this again," Obama said. "Apparently I wasn't clear enough this morning on my position with respect to the war in Iraq."

The evolving situation in Iraq has, in fact, tested both candidates. McCain, whose support for the unpopular war helped him win the Republican primary, now finds that he must explain his position to a general electorate largely weary of the war. And for Obama the suggestion that he was having second thoughts about a central premise of his candidacy holds particular perils.

In his second news conference on Thursday, Obama laid out his proposal in less ambiguous terms. "Let me be as clear as I can be," he said.

"I intend to end this war. My first day in office I will bring the Joint Chiefs of Staff in, and I will give them a new mission, and that is to end this war - responsibly, deliberately, but decisively. And I have seen no information that contradicts the notion that we can bring our troops out safely at a pace of one to two brigades a month, and again, that pace translates into having our combat troops out in 16 months' time."

Some foreign policy experts say that both candidates may have to adjust their stances once in office - McCain, because strains on the military may make it impossible to station as many troops there for as long as he likes, and Obama, because the threat of backsliding may force him to slow the pace of withdrawal.

This article on the national political campaigns in the United States is from The New York Times. It was specially selected and prepared by the editors of The New York Times News Service.