WASHINGTON (Reuters) - President Barack Obama said his decision to double the number of U.S. military advisers in Iraq marked a new phase in the campaign against Islamic State and was not an indication his strategy in the region had failed.

Obama, in an interview on CBS' "Face the Nation" broadcast on Sunday, said the first phase was getting an Iraqi government in place that was inclusive and credible.

He said sending in 1,500 additional American troops also signified a shift from a defensive strategy to an offensive one. The decision was announced on Friday.

"The air strikes have been very effective in degrading ISIL's capabilities and slowing the advance that they were making," Obama said, according to a CBS transcript. "Now what we need is ground troops, Iraqi ground troops, that can start pushing them back."

The U.S. president did not rule out sending more troops back to the region. "You know, as commander in chief I'm never going to say never," he told CBS.

But Obama said his military commanders believe fewer troops would be needed over time as coalition members join the United States in training and assisting Iraqi troops.

U.S. air strikes on Saturday destroyed an Islamic State convoy near the Iraqi city of Mosul, but U.S. officials said it was unclear whether the group's top commander, Abu Bakr al-Baghdadi, had been in any of the 10 targeted vehicles.

The decision to send more troops came five months after Islamic State seized much of northern Iraq. The militant group has also seized territory in Syria, where the U.S. is leading air strikes targeting the militants.

Obama's 2012 withdrawal of combat troops from Iraq has been sharply criticized by some Republicans, including U.S. Senator John McCain, as having left the country to spin into sectarian strife and chaos.

On Sunday television news shows, Republican and Democratic lawmakers said they believed Congress should have a say in troop decisions like the one Obama announced last week.

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"I think at some point we are going to be debating an authorization on the use of military force," Senator John Thune said on CNN's "State of the Union."

Democratic Senator Chris Murphy said he did not think Obama could authorize 1,500 troops without Congress acting, saying he and other lawmakers want evidence that Iraq's government will be able to sustain U.S. advances this time.

"We had hundreds of thousands of troops inside Iraq over the course of a decade trying to train Iraqi armed forces. They got overrun in a period of weeks by a relatively unorganized force," he said on CNN. "I want to make sure that this is a realistic mission.

(Reporting by Doina Chiacu and Valerie Volcovici; Eiting by Frances Kerry and Dominic Evans)