The US top diplomat headed for talks with the Syrian opposition and its key backer Saudi Arabia Friday after the White House asked lawmakers for $500 million to train and equip vetted rebels.
The move would mark a significant escalation of US involvement in the three-year-old civil war in Syria, which is now increasingly interlinked with a jihadist-led Sunni Arab insurgency in neighbouring Iraq.
Saudi King Abdullah, who will meet Secretary of State John Kerry in the Red Sea city of Jeddah, has long called for greater US military support for the Syria rebels, whom the Sunni kingdom has long backed.
He has also been an outspoken critic of Iraqi Prime Minister Nuri al-Maliki whose Shiite-led government has seen insurgents sweep up a huge swathe of territory north of Baghdad, including second city Mosul, since June 9.
Riyadh accuses Maliki of excluding Iraq's Sunni Arab minority and has played down Western concerns that the insurgents are led by jihadists.
Maliki has hit back, accusing both Saudi Arabia and its neighbour Qatar of supporting terrorism.
The White House made clear in its request to Congress that the military aid it was proposing for Syria would be for vetted rebels only, to address concerns that US weapons supplied to the battlefield might fall into the hands of jihadists on both sides of the Syria-Iraq border.
The ultra-hardline Islamic State of Iraq and the Levant, which has spearheaded the militant offensive north of Baghdad, has also played a major role in the revolt in Syria against President Bashar al-Assad's rule.
The White House said it intends to "ramp up US support to the moderate Syrian opposition" and Kerry was to hold talks in Jeddah with the leader of the Western-backed Syrian National Coalition, Ahmad Jarba.
- 'Lethal support' -
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Washington has been studying options for providing additional assistance to rebel forces beyond the existing aid, which includes mainly "non-lethal" support.
While US officials normally refuse to publicly comment on details of training for opposition groups, Obama's National Security Adviser Susan Rice acknowledged early this month that the Pentagon was providing "lethal and non-lethal support" to Syrian rebels.
About $287 million in mainly non-lethal support has been cleared for the rebels since March 2011, and the CIA has participated in a secret military training programme in neighbouring Jordan for the moderate opposition.
Washington has been increasingly concerned that the jihadists' battlefield role on both sides of the Iraq border will play into the hands of the Assad regime.
Those fears were highlighted Thursday when Maliki "welcomed" Syria government air strikes against ISIL fighters who control both sides of the Al-Qaim crossing on their common border.
Washington responded that military action by Assad's government would not be "in any way helpful to Iraq's security."
Maliki's government had requested US air strikes against the militants battling his security forces, but Washington has offered only military advisers, the first of whom have begun work.
Maliki has turned instead to Belarus and Russia from whom it has purchased several used Sukhoi fighters to boost its ability to strike the militants from the air.
US F16 fighter jets that Baghdad ordered from Washington remain in the United States.
And Obama, who made his political career out of opposition to Washington's bloody 2003-2011 war in Iraq, reiterated Thursday that there would be no return of US combat troops.
"We've got to pay attention to the threats that are emanating from the chaos in the Middle East, although I want to be very clear: We're not sending combat troops into Iraq," he said.
"They're going to have to contribute to solving their own problems here, although we'll protect our people and we'll make sure that we're going after terrorists who could do us harm."