Syria and Iran have condemned a move by the US to give non-lethal aid to rebels fighting to topple the Syrian president, Bashar al-Assad, accusing Washington of double standards.

"I do not understand how the United States can give support to groups that kill the Syrian people," the Syrian foreign minister, Walid al-Moualem, told a news conference on Saturday 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 $60m to help the civil opposition provide basic services such as security, education and sanitation.

Iran's Salehi said the US 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 US, Europe and Middle Eastern countries generally back the opposition.

Some European countries are expected to break with Washington and start supplying the Syrian rebels with weapons in the next few months, the representative of the Syrian opposition in Britain said.

The National Coalition's London representative, Walid Saffour, predicted that by the next meeting of the western and Arab Friends of Syria group in Turkey, due in late spring or early summer, "there will be a breakthrough that will end the restrictions of the European countries".

"This would be for the ammunition we require, the quality weapons we need to deter the Syrian regime from using aeroplanes and Scud missiles to bomb villages and bakeries," Saffour said. "We on the ground are advancing steadily but we are suffering from a lack of ammunition. We expect that to change at the next Friends of Syria meeting in Istanbul."

Another opposition figure involved in supplying the rebels said there had been a noticeable relaxation in recent days of the strict restrictions the US and Turkey had put on arms flows over the Turkish border. He claimed a Syrian army helicopter and a Mig warplane had been shot down in the past two days, for the first time by imported missiles.

"These were not weapons that had been captured from Syrian army bases as before. These were released from the Turkish warehouses. These are weapons the opposition had purchased previously but had not been allowed to take across the border," the opposition source said.

"Before, 23mm was the maximum calibre for anti-aircraft guns permitted and we were allowed to bring in RPGs [rocket-propelled grenades] but not armour-piercing shells. But there is a major shift on the ground now. The policy is changing.

"I think the shift in American attitudes goes far beyond the official reports. I think that Washington knows it can no longer allow the to problem fester."