US Secretary of State Hillary Clinton was in Istanbul for talks on the Syrian conflict on Saturday as fighting for control of the strategic city of Aleppo raged just 50 kilometres (30 miles) from Turkey's southern border.
Clinton was to discuss with Turkish leaders boosting support for the Syrian opposition after Washington gave up on securing action at the UN Security Council against President Bashar al-Assad's regime, a US official said.
Turkey has given shelter to hundreds of defecting Syrian soldiers, including more than 30 generals, who have formed the kernel of the rebel army, now boosted by many more civilian volunteers, that is fighting government troops in Aleppo and other cities.
Fierce clashes in the commercial capital since Wednesday, when the army launched a ground offensives to retake rebel-held neighbourhoods, have exacted a mounting toll on civilians in the city of some 2.7 million people.
On Friday, a shell crashed into a bakery in the eastern Tariq al-Bab district of the city as hundreds of desperate residents queued for bread, AFP correspondents reported.
A dozen people were killed, three of them children, and 20 more wounded.
City wide, the Britain-based Syrian Observatory for Human Rights said it could confirm 21 people killed on Friday, a full 19 of them civilians.
They were among 187 people killed in violence across Syria, among them 111 civilians, the watchdog said.
The Observatory said the army launched a fierce bombardment early Saturday of the rebel-held Sukari district of south Aleppo which appeared to be in preparation for a ground assault.
Troops backed by artillery stormed the nearby Salaheddin district on Wednesday in the first stage of its offensive to recapture areas of the city taken by the rebels since July 20, prompting the insurgents to retreat under heavy fire.
A rebel commander said on Saturday that his fighters were still putting up strong resistance around Salaheddin.
"Fierce fighting has continued without respite for the past 24 hours as the army tries to push us out of the neighbourhood," Abdel Qader Saleh told AFP by telephone.
State media said the army repelled a rebel attack on Aleppo's international airport on Friday.
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"Mercenary terrorists" had tried to attack it but the "army hit back and killed most of them," the official SANA news agency reported.
In Istanbul, Clinton was to meet President Abdullah Gul, Prime Minister Recep Tayyip Erdogan and Foreign Minister Ahmet Davutoglu, as well as Syrian opposition members and refugees.
Turkey, once a close ally of Syria, has become a vocal opponent of the regime since it launched a brutal crackdown on dissent in March last year.
Relations hit an all-time low after a Turkish fighter jet was shot down by Syria in June, killing its two-man crew and leading Ankara to brand Damascus a "hostile" opponent.
In November, Turkey joined Arab League sanctions, freezing Syrian government financial assets, imposing a travel ban on senior Syrian officials and cutting off transactions with the country's central bank.
China and Russia vetoed three attempts to secure Security Council action against Assad's regime.
"So after that... we made clear that we were shifting from New York to a focus on supporting the opposition in its efforts to hasten the day that Assad falls and to begin in earnest planning for the day after Assad falls," a State Department official said.
"The secretary was very clear that we don't want to put a date on Assad's departure, because we can't," the official said,
"But it is our strong conviction that it will come and that the international community needs to be prepared to support the Syrians themselves as they deal with all of the challenges that will come with actually effectuating a transition to a new Syria.
"There are challenges related to securing, of course, weapons inside Syria to ensure they don't fall into the wrong hands.
"And there are humanitarian challenges related to the need to provide basic subsistence to displaced Syrians both inside and outside Syria."
Clinton's talks come a day after Washington announced sanctions against Syrian state oil company Sytrol for trading with Iran, in a bid to starve both Damascus and its Tehran ally of much-needed revenue.
The US Treasury also said it was adding the Lebanese Shiite militant group Hezbollah, which has close ties with both Iran and Syria, to a blacklist of organisations targeted under Syria-related sanctions.
Washington already classes Hezbollah a "terrorist organisation" and it is under US sanctions, but Friday's move explicitly ties the group to the violence in Syria.