Hillary Clinton, the US secretary of state, is holding talks on the conflict in Syria with Turkish leaders as deadly clashes between Syrian troops and rebels continue in the northern city of Aleppo.
Clinton arrived in Istanbul early on Saturday to have a "lengthy and in-depth conversation" with Turkey's president, prime minister and foreign minister to discuss a three-pronged strategy, a US official said.
The first aspect is "how we judge the effectiveness of what we are doing in terms of supporting the opposition," the official said, adding that pressure and isolation of the regime was part of the strategy.
The third strategy, the US official said, is built on a transition plan out of the "strong conviction" that Bashar al-Assad's, the Syrian president, days are numbered and that the international community needs to be prepared to support Syrians.
Washington on Friday announced sanctions against Syrian state oil company Sytrol for trading with Iran, in a bid to help starve the regime of energy supplies.
In Istanbul, Clinton is expected to announce an additional $5.5m in aid for those fleeing fighting that monitoring groups say has now claimed more than 21,000 lives.
Turkey is currently home to more than 50,000 refugees living in camps along the Syrian border.
Deadly battles rage
The UN on Friday said there were more than 6,000 new arrivals in Turkey this week alone, many from the besieged Syrian city of Aleppo and surrounding villages, while others came from Idlib and Latakia.
In Aleppo, dozen of people, including three children, were killed and 20 wounded when a shell crashed into a bakery as hundreds of residents queued for bread at a bakery in the eastern Tariq al-Bab district, AFP news agency correspondents said on Friday.
In-depth coverage of escalating violence across Syria
Meanwhile, Syrian state media said regime troops repelled a rebel attack on Aleppo's international airport.
"Mercenary terrorists" had tried to attack it but the "army hit back and killed most of them," the Sana news agency reported on Friday.
In the Damascus suburb of Deir al-Asafir, activists on Friday reported clashes between government forces and rebels and amateur video posted on the internet showed smoke rising after an attack there.
Syrian troops say they have purged rebels from the capital after intense, week-long battles last month. But rebels continue to stage hit-and-run attacks and are active in the suburbs around the city.
Clashes at Jordanian border
Meanwhile, Fighting spilled across the border, as Jordanian and Syrian forces clashed in the Tel Shihab-Turra border region on Friday.
A source in Jordan confirmed the fighting but said there appeared to have been no Jordanian deaths.
"The Syrian side fired across the border and fighting ensued," the source, speaking on condition of anonymity, told the Reuters news agency.
A Syrian opposition activist who witnessed the fighting said armoured vehicles were involved in the clash in Tel Shihab-Turra, about 80km north of the Jordanian capital Amman, which occurred after Syrian refugees tried to cross into Jordan.
Syria's long border with Jordan has been an escape route for opponents of Assad, including Prime Minister Riad Hijab, who defected this week.