US President Barack Obama has authorised air strikes against Islamic State group targets inside Syria for the first time, pledging to destroy its fighters "wherever they exist".

In an address to the nation on Wednesday, Obama also announced an expansion of strikes in Iraq, saying he would be dispatching nearly 500 more US troops to the country to assist its besieged security forces.

Obama called on Congress to authorise a programme to train and arm rebels in Syria who are fighting both the Islamic State group and Syrian President Bashar al-Assad.

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Saudi Arabia, a crucial US ally in the Middle East, offered to host the training missions, part of Obama's effort to persuade other nations to join with the US in confronting the self-declared jihadist fighters.

"This is not our fight alone," Obama declared.

"American power can make a decisive difference, but we cannot do for Iraqis what they must do for themselves, nor can we take the place of Arab partners in securing their region."

"Our objective is clear: We will degrade and ultimately destroy ISIL (Islamic State) through a comprehensive and sustained air strikes strategy.''

Obama adamantly ruled out the prospect of putting American troops in combat roles on the ground in Iraq or Syria.

"President Obama is trying to send a message to the public that this is not another full-blown war. US officials are making a concerted effort to say 'this is like Yemen and Somalia'," Al Jazeera's Patty Culhane, reporting from the White House, said.

"Whether in fact it will be like Yemen and Somalia remains to be seen. In those countries, there have been mostly unpublicised, targeted, very occasional drone strikes," she said.

"What the president is talking about now is a constant series of air strikes and a much deeper involvement with Iraq and Syria."

The Islamic State group has taken control of large parts of Iraq and Syria. The group's brutal campaign has included beheadings of rivals and killings of hundreds of members of Iraq's minorities.

In recent weeks, the Islamic State group has released videos depicting the beheading of two American journalists in Syria.

The violent images appear to have had an impact on a formerly war-weary public, with multiple polls in recent days showing that the majority of Americans support air strikes in both Iraq and Syria.

Using 'force authorisation'

The US began launching limited air strikes against Islamic State targets in Iraq earlier this summer.

But officials said Obama was waiting for Iraq to form a new government - a step it took on Tuesday - before broadening the effort.

Officials said strikes in Iraq would now be wide-ranging and extend into Syria. Obama plans to proceed with those actions without seeking new authorisation from Congress.

Instead, officials said Obama will act under a use of force authorisation Congress passed in the days after 9/11 to give President George W Bush the ability to go after those who perpetrated the attacks.

Obama has previously called for that authorisation to be repealed, he has also used the measure as a rationale to take strikes against terrorist targets in Yemen and Somalia.

Earlier on Wednesday, John Kerry, US secretary of state, said the international community will not sit and watch the Islamic State grow. He identified Iraq as a key partner in the fight against the group.

Source: Al Jazeera and agencies