The United States will provide food and medical supplies to Syrian rebels for the first time, according to US secretary of state John Kerry.
The Obama administration will also grant an additional $60m in aid to opponents of President Bashar al-Assad.
Kerry signalled Washington's unprecedented support for the opposition on the sidelines of an international conference on Syria in Rome. European nations were expected to announce their intention to provide fresh assistance to rebels fighting against Assad.
A European diplomatic source told Reuters that Western and Arab officials would meet representatives of the Syrian National Coalition in Istanbul to discuss military and humanitarian support.
"The meeting aims to work directly with opposition structures to increase the level of support," the source said. Both civilian and military representatives of the Syrian opposition would attend.
Kerry said: "No nation, no people, should live in fear of their so-called leaders." Assad, he added, was "out of time and must be out of power".
"For more than a year, the United States and our partners have called on Assad to heed the voice of the Syrian people and to halt his war machine," Kerry said. "Instead, what we have seen is his brutality increase.
"The United States' decision to take further steps now is the result of the brutality of superior armed force propped up by foreign fighters from Iran and Hezbollah."
The Obama administration believe that providing aid would thwart extremists propaganda in the Syrian opposition. Washington has already provided $385m in humanitarian aid to Syria's war-weary population and $54m in communications equipment, medical supplies and other non-lethal assistance.
But this is the first time that the US has granted provisions directly to rebel fighters. The food rations and medical supplies will be delivered through the opposition's military council.