US President Barack Obama said he has authorised air strikes against Isis in Syria and pledged more attacks against the Islamic State in Iraq.

Addressing the nation on the US strategy to defeat the Isis terrorists, he said: "This is a core principle of my presidency: if you threaten America, you will find no safe haven."

Making it clear that this was not an all-out war involving combat forces on the ground, Obama said: "We will degrade, and ultimately destroy ISIL through a comprehensive and sustained counter-terrorism strategy,"

Obama said he has approved sending additional 475 military advisers to Iraq. There are around 1,700 US military personnel in the region at present.

Noting that "Isis is not Islam", but a terrorist organisation that had killed and tortured women and children in Iraq and Syria, Obama said there was need for a comprehensive and sustained counterterrorism strategy to curb the spreading of the group beyond the region.

The US has conducted more than 150 airstrikes against the Islamic State in Iraq since 9 August.

US diplomatic efforts to consolidate support and broaden the coalition against Isis saw Secretary of State John Kerry on a mission to convince Sunni leaders in Jordan and Saudi Arabia to join the US and its allies.

Defence Secretary Chuck Hagel and Lisa Monaco, the homeland security adviser, have also left for the region, reports CNN.

Congress support

Support for US action in Iraq seems to be unanimous among the Congress and diplomats pushing for an expanded fight against the Isis.

"There's been no pushback against the Islamic State and they have made breathtaking advances. We haven't seen anything like this since Hitler and the blitzkrieg in World War II," said Rep Michele Bachmann (R-Minn), who sits on the House Intelligence Committee.

"They're going to recruit people in other countries, including our country, who could conduct a terrorist attack here," said Sen John Hoeven (R-N D). "So yes, they are a clear threat to us."

Former US ambassador Ryan Crocker, who served in Syria, Iraq, Afghanistan and Pakistan, has talked of the Isis posing a bigger threat to America than al-Qaeda.

"They are more numerous, they are better armed, they are far better financed, they are better experienced, and perhaps most critically there are several thousand of them who hold Western passports, including American passports," Crocker said.

However, perception of threat to the US seems to vary. While Hagel sees an imminent threat from Isis to US interests abroad, the FBI and the Department of Homeland Security say that the group poses "no specific or credible terror threats to the US homeland."

The Office of Director of National Intelligence said the Islamic State's "ability to carry out complex, significant attacks in the West is currently limited", but added that "the United States is not immune".