"I do not understand how the United States can give support to groups that kill the Syrian people," Syrian Foreign Minister Walid al-Moualem said at a news conference in Tehran with Ali Akbar Salehi, Iran's foreign minister.
"This is nothing but a double-standard policy ... One who seeks a political solution does not punish the Syrian people."
The United States said on Thursday it would for the first time give non-lethal aid to Syrian rebels, describing the aid as a way to bolster the rebels' popular support.
The assistance will include medical supplies, food for rebel fighters and $60 million to help the civil opposition provide basic services like security, education and sanitation.
Iran's Salehi said the U.S. move would prolong the Syrian conflict, an uprising-turned-civil war in which 70,000 people have been killed.
"If you really feel sorry about the ongoing situation in Syria you should force the opposition to sit at the negotiation table with the Syrian government and put an end to bloodshed," he said.
"Why do you encourage the opposition to continue these acts of violence?"
Iran and Russia support Assad, while the United States and its allies generally back the opposition.
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