WASHINGTON (AP) — President Barack Obama says the United States will expand its offensive against Islamic State militants, authorizing military airstrikes in Syria. The declaration intensifies the U.S. effort to confront the extremist group that has overrun parts of Iraq and Syria and recently beheaded two Americans.

In a prime-time address to the nation, Obama said the U.S. will confront Islamic State militants "wherever they exist."

Obama administration officials were silent on when such strikes could begin, saying they did not want to telegraph such actions.

The administration has already targeted Islamic State forces in Iraq at the invitation of the Iraqi government. But in Syria, the U.S. would have to act without permission from Syrian President Bashar Assad. The U.S. has supported Assad's ouster and has backed moderate rebel forces.

A U.S. official says Saudi Arabia has agreed to open its bases for the training of moderate Syrian rebel forces as part of an effort to better equip them against the threat of Islamic State militants.

The decision by the Saudis signals a significant new role for the influential Sunni kingdom in the fight against the Islamic State group.

President Barack Obama is asking Congress for authorization to arm and train Syrian fighters.

The use of moderate Syrian rebels is a key component of Obama's strategy because the U.S. considers the government of President Bashar Assad to be illegitimate.

The rebel fighters are essentially in a two-front conflict, fighting both Islamic State militants and Assad's forces.

President Barack Obama is sending 475 more U.S. troops into Iraq to supplement an existing American military presence that is helping Iraqi security forces battle Islamic State militants and secure diplomatic facilities.

Obama was announcing the additional forces Wednesday as part of an expanded offensive against the Islamic State group in Iraq and in Syria.

The new troops would join more than 1,000 U.S. military personnel in Iraq and would be embedded with Iraqi security forces and help identify targets for U.S. military airstrikes against the extremist group.

The U.S. already has more than 750 U.S. military personnel supporting diplomatic security at facilities in Baghdad and nearly 300 at joint operating centers in Baghdad and Irbil (ur-BEEL').