Syria's main Western-backed opposition group on Thursday welcomed President Barack Obama's authorization of U.S. airstrikes targeting Islamic State of Iraq and Syria (ISIS), saying it stands "ready and willing" to partner with the international community to defeat the extremists.

But the Syrian National Coalition said that equally important was the realization that fighting ISIS alone is not enough and should be coupled with degrading and ultimately removing President Bashar Assad's regime.

"The Syrian Coalition ... stands ready and willing to partner with the international community not only to defeat ISIS but also rid the Syrian people of the tyranny of the Assad regime," said the Coalition's chief Hadi Bahra.

In a prime-time address to the nation from the White House, Obama announced he was authorizing U.S. airstrikes inside Syria for the first time, along with expanded strikes in Iraq as part of "a steady, relentless effort" to root out Islamic State extremists and their spreading reign of terror.

He also urged Congress anew to authorize a program to train and arm Syrian rebels who are fighting both the Islamic State militants and Syrian President Bashar Assad.

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A member of the Syrian Arab Red Crescent helps an injured man after what activists said were airstrikes on Wednesday by forces loyal to Syria's President Bashar al-Assad, in the Duma neighbourhood of Damascus. (Bassam Khabieh/Reuters)

"We will hunt down terrorists who threaten our country, wherever they are," Obama declared. "That means I will not hesitate to take action against [ISIS] in Syria, as well as Iraq."

Potential to strengthen Assad's hand

Obama did not say when U.S. forces would begin striking at targets inside Syria.

The U.S. began launching limited airstrikes against ISIS targets in Iraq earlier this summer at the request of that country's former prime minister. The Sunni extremists have seized roughly a third of Iraq and Syria, declaring a self-styled caliphate in areas under their control.

Obama ruled out any partnership with Assad in the fight against the Islamic State militants, saying he will "never regain the legitimacy" he has lost.

"Instead, we must strengthen the opposition as the best counterweight to extremists like [ISIS], while pursuing the political solution necessary to solve Syria's crisis once and for all," he said.

Bahra said mainstream Syrian rebels desperately need the kind of support that would enable it to form a reliable and well-equipped force to fight the extremists.

"Today, we are one step closer to achieving that goal," he said.

A year ago, Obama gave a similar speech in which he was widely expected to announce the U.S. would be launching punishing airstrikes against Assad's forces, after blaming them for a deadly chemical weapons attack near Damascus. Obama backed down at the last minute.

Ironically, Obama has now authorized airstrikes not against Assad, but against a group committed to his removal from power. In doing that, the U.S. runs the risk of unintentionally strengthening Assad's hand, potentially opening the way for the Syrian army to fill the vacuum left by the extremists.