By Jeff Mason

MACDILL AIR FORCE BASE Fla. (Reuters) - Some U.S. military advisers could end up in front-line positions in the fight against Islamic State militants, the White House said on Wednesday, although President Barack Obama vowed that America would not fight another ground war in Iraq.

The House of Representatives gave a green light to a key plank of Obama's strategy against Islamic State, approving authorization for the Pentagon to train and arm moderate Syrian rebels to fight the radical Islamist militants. The legislation now goes to the Senate.

Obama, who has spent much of his presidency distancing himself from the Iraq War, stressed that air strikes would be the central U.S. contribution to the fight against Islamic State in Iraq and Syria, along with coordinating a coalition that he said now includes more than 40 countries.

"I want to be clear. The American forces that have been deployed to Iraq do not and will not have a combat mission," Obama said in a speech at MacDill Air Force Base in Tampa.

His spokesman Josh Earnest said some U.S. military advisers could be deployed to forward positions to help direct Iraqi security forces and call in air strikes, but said they would "not be personally or directly engaging the enemy."

The possibility of U.S. soldiers operating in forward positions with local forces raised concerns that Washington, which withdrew its forces from Baghdad in 2011 after eight years of conflict, could once again be drawn into a bigger engagement.

On Tuesday General Martin Dempsey, chairman of the U.S. military's Joint Chiefs of Staff, had raised the possibility of such eventual forward deployments during a committee hearing.

NO U.S. GROUND WAR

"If General Dempsey determined that it may be necessary to forward deploy some of the American advisers, then he will bring that option to the president, and the president said that he would consider it on a case by case basis," Earnest told reporters traveling with Obama on Air Force One.

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The House of Representatives approved the measure authorizing the Syrian rebel training plan despite some resistance from Republicans and Democrats, some of whom expressed concern about being sucked back into war in Iraq.

The Senate is expected to also pass the authorization by the end of the week. It will last until Dec. 11.

Lawmakers of both parties voiced skepticism about the White House strategy when they questioned one of its key architects, Secretary of State John Kerry. Members of the Foreign Relations Committee raised doubts about the ability of local Iraqi and Syrian forces to fight Islamic State and questioned whether Obama had the legal authority to carry out a lengthy operation.

The United States has launched more than 160 air strikes against Islamic State targets in Iraq and Obama has authorized similar attacks against the group's strongholds in Syria.

More than 1,600 American advisers have been dispatched to help Iraqi forces but Obama does not want them to get involved in ground combat to avoid a repeat of the Iraq war begun by his Republican predecessor, George W. Bush.

In testimony to a Senate committee, Dempsey had outlined scenarios in which he might recommend having U.S. troops do more, potentially accompanying Iraqis during complicated offensives, such as a battle to retake the northern city of Mosul from Islamic State fighters.

LIMITS OF AIR POWER

Former Defense Secretary Robert Gates told CBS's "This Morning" that Dempsey's remark made sense because air power alone has its limitations. "The reality is they're not going be able to be successful against ISIS strictly from the air," he said, using an acronym for Islamic State.

Obama was to meet his top national security advisers at the White House later on Wednesday to discuss a meeting of leaders at the U.N. General Assembly next week on how to get control of foreign fighters who have sworn allegiance to Islamic State and could return to their home countries to attack civilians.

The FBI director and the heads of the Department of Homeland Security and the National Counter terrorism Center said that people radicalized by online propaganda from Islamic State and other groups posed the main terrorist threat to America.

They told a congressional committee that while there was no evidence that Islamic State planned an attack on American soil, its vigorous propaganda and sophisticated online recruitment efforts created a clear potential threat.

(Additional reporting by Roberta Rampton, Patricia Zengerle and David Lawder; Writing by Steve Holland; Editing by David Storey and Howard Goller)