* Deepens U.S. role in Syria's civil war
* Syrian rebels, U.S. lawmakers skeptical about training program (Rewrites throughout after Pentagon confirms start of military training program)
By Phil Stewart and Dasha Afanasieva
WASHINGTON/ISTANBUL, May 7 (Reuters) - The United States has begun a long-awaited program to train Syrian fighters to go into combat against Islamic State, the Pentagon said on Thursday, deepening America's role in Syria's civil war after eight months of airstrikes against the Sunni militants.
Defense Secretary Ash Carter said the military was starting small, training a first group of just 90 Syrians, who would be paid a stipend and could expect some still-undefined support once they return to the battlefield.
A spokesman for the government of Jordan said the training began there several days ago and U.S. and Middle Eastern sources told Reuters the training would soon start at another site in Turkey.
Syrian rebels and members of the U.S. Congress are deeply skeptical, with some lawmakers saying the program is too small and slow. The Pentagon forecasts it will take three years to train and arm more than 15,000 opposition forces.
Carter acknowledged that it would be a few months before even the first group of 90 fighters would be deployed and he described the effort as advancing with tremendous caution, in part to limit the risks that U.S.-trained fighters would commit human rights violations.
"We're starting with the people that we have that we've vetted very carefully," Carter told a Pentagon news briefing.
"We expect that to be successful and therefore to grow. But you have to start somewhere, and this is where we're starting."
The Obama administration says the program aims only to target Islamic State forces, since the United States is not at a war with Syria. But critics say that theoretical limitation is unlikely to withstand the realities of Syria's messy civil war.
U.S.-trained Syrian fighters, experts say, are likely to come in contact with Syrian government forces eventually. And the priority of U.S. allies in the region, including Saudi Arabia and Turkey, is to topple Syrian President Bashar al-Assad.
Even as the Pentagon announced the start of training, it acknowledged it was still unclear what it would do to support the rebels if they intentionally or unintentionally engage in fighting with Syrian government forces.
"We have not determined yet all of the rules of engagement ... but we have acknowledged that we have some responsibility to support them," Carter said.
Part of the U.S. strategy, according to Obama administration documents seen by Reuters, is to pressure Assad by steadily increasing the opposition's prowess and territory under its control.
Proponents of the U.S. military program note Assad is already facing growing pressure after government forces endured a series of setbacks on the battlefield and Islamic State fighters edge closer to Assad's stronghold in the coastal areas.
General Martin Dempsey, the top U.S. military officer, said he believed Assad's "momentum has been slowed."
"I do believe that the situation is trending less favorably for the regime. And if I were him, I would find the opportunity to look to the negotiating table," said Dempsey, chairman of the U.S. military's Joint Chiefs of Staff.
The civil war has killed 220,000 people and displaced millions since 2011, despite repeated diplomatic efforts to resolve it. The United Nations began a new push this week. (Additional reporting by Suleiman Al-Khalidi in Amman and David Alexander in Washington; Editing by Doina Chiacu, Bernadette Baum, Grant McCool)