President Barack Obama's top foreign policy advisor Susan Rice on Friday said Washington was providing "lethal and non-lethal" support to select members of the Syrian opposition, offering more detail than usual on US assistance.
Top Obama administration officials typically decline to say exactly what equipment, arms or ammunition the United States is providing to moderate Syrian opposition forces.
But President Barack Obama said in a major foreign policy speech last week that the United States would "ramp up" support for rebels fighting President Bashar al-Assad.
National Security Advisor Susan Rice said in an interview with CNN while she was traveling with Obama to D-Day 70th anniversary celebrations in Normandy that she was heartbroken about the carnage in Syria's civil war.
"That's why the United States has ramped up its support for the moderate vetted opposition, providing lethal and non-lethal support where we can to support both the civilian opposition and the military opposition."
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Officials normally publicly refuse to comment on exactly what they are doing to train opposition groups.
National Security Council spokeswoman Caitlin Hayden declined to say whether Rice was announcing a new US policy by apparently being more open on US assistance.
"We're not in a position to detail all of our assistance, but as we've made clear, we provide both military and non-military assistance to the opposition," Hayden said.
Signs of a deepening commitment to Syrian rebels come three weeks after Obama met the head of the opposition National Coalition, Ahmad Jarba in Washington last month.
Officially, US support for rebel fighters in Syria has been limited to non-lethal aid amounting to $287 million, though the CIA reportedly participates in a secret programme to train moderate rebels in Jordan.
Opposition leaders are particularly dismayed that the United States has balked at providing anti-aircraft missiles to rebels, fearing they could fall into the wrong hands.
The Wall Street Journal has reported that Obama is ready to sign off on training missions for selected rebel groups, to counter the rising power of Al-Qaeda-linked extremists.