The Obama administration set new, largely symbolic, sanctions on Friday on Syria’s state-run oil company and the Hizbollah group, moves designed to underscore Iran’s key role in propping up the Syrian regime over the span of its civil war.

State Department spokesman Patrick Ventrell said the penalties against energy firm Sytrol come after it delivered $ 36 million worth of gasoline to Iran in April.

At the same time, Tehran was “actively advising, supplying, and assisting the Syrian security forces and regime-backed militias that are carrying out gross human rights abuses against the Syrian people.”

Meanwhile, the Treasury Department targeted Hizbollah for “training, advice and extensive logistical support to the government of Syria’s increasingly ruthless efforts to fight against the opposition.”

It also blamed the Lebanese group for co-ordinating Iranian assistance to the Syrian government.

“Hizbollah’s extensive support to the Syrian government’s violent suppression of the Syrian people exposes the true nature of this terrorist organisation and its destabilising presence in the region,” the Treasury Department’s sanctions chief, David S Cohen, said.

“Long after the Assad regime is gone, the people of Syria and the entire global community will remember that Hizbollah, and its patron Iran, contributed to the regime’s murder of countless innocent Syrians.” Ventrell said Iran’s support for the Assad regime, including equipment to monitor opposition activity on the Internet, was “completely unjustifiable.”

He said that Iran fears losing its only remaining ally in the Middle East, Syria.

“Today’s sanctions action sends a stark message: the United States stands resolutely against sales of refined petroleum product to Iran and will employ all available measures to bring it to a halt,” he added. “Any business that continues to irresponsibly support Iran’s energy sector or helps facilitate either nation’s efforts to evade US sanctions will face serious consequences.”

Syrian troops and rebels fought fierce battles on Friday in the city of Aleppo, where several people died when a shell crashed into a bakery as hundreds queued for bread, correspondents said.

They said around a dozen people were killed and 20 wounded at the bakery in the increasingly desperate city.

At least three children were among the dead in the eastern Tariq Al Bab district of Syria’s commercial capital.

And troops repelled a rebel attack on Aleppo’s international airport, state news agency Sana reported.

“Mercenary terrorists” had tried to attack it but the “army hit back and killed most of them.”

Rebels vowed to fight on in Aleppo, a day after being driven out of a key district under heavy shellfire. In the latest clashes, Aleppo’s historic Citadel, part of a Unesco-listed world heritage site, was heavily damaged by bombing, according to the opposition.

The violence raged on as world powers prepared to name veteran Algerian diplomat Lakhdar Brahimi as their new envoy to seek a peaceful and politically workable end to a 17-month uprising that has cost more than 21,000 lives.

A rebel commander, Hossam Abu Mohammed, said his men were still fighting in parts of Aleppo’s southwestern district of Salaheddin after most fled on Thursday in the face of heavy bombing and advancing troops.

“We will not let Salaheddin go,” the Free Syrian Army’s (FSA) Abu Mohammed said by telephone on the third day of a government offensive to take the city.

The army again bombed parts of Salaheddin, as well as the Sakhur and Hanano districts in the northeast, the Syrian Observatory for Human Rights said, adding that five civilians were among 56 people killed nationwide.

A rising tide of civilians fleeing Syria’s violence is hitting four neighbouring countries where almost 150,000 are being helped in camps run by the UN refugee agency and its partners, officials said on Friday.

That figure counts only Syrians who have registered or are in the process of registering as refugees. Officials acknowledge the real number of Syrian refugees is likely above 200,000 since tens of thousands are believed to have not yet registered with authorities.

Spokesman Adrian Edwards told reporters in Geneva that the UN refugee agency’s offices in Lebanon, Turkey, Jordan and Iraq have all reported big increases this week in the number of registrants.