AZAZ, Syria -- Syrian rebels were running low on ammunition and guns Friday as government forces tried to consolidate their control over Aleppo, the country's largest city and a deadly battleground for more than two weeks.

The seemingly intractable, 17-month-old conflict in Syria has defied all international attempts to calm the bloodshed. Rebels and activists said Friday they have had enough of diplomacy and appealed to the international community to send weapons.

"The warplanes and helicopters are killing us. They're up there in the sky 15 hours a day," said Mohammad al-Hassan, an activist in Aleppo's Salaheddine district, the main rebel stronghold in the city.

"It's warplanes against Kalashnikovs, tanks fighting against rifles," he said. "I don't know how long this situation can be sustained."

As Syrian soldiers bombarded rebel positions in Aleppo from the ground and air, diplomats said former Algerian foreign affairs minister and longtime UN official Lakhdar Brahimi has emerged as a strong candidate to replace Kofi Annan as UN peace envoy to Syria.

Annan announced his resignation last week, ending a frustrating six-month effort that failed to achieve even a temporary cease-fire as the country descended into civil war. Activists say some 20,000 people have been killed.

Also Friday, the United States announced sanctions on Hezbollah for providing support to Syrian President Bashar Assad's regime -- a symbolic move, as Washington already has designated the Lebanese militant group a terrorist organization.

Still, the sanctions emphasized how Syria's alliance with Hezbollah -- and with the group's patrons in Iran -- means the conflict has the potential to escalate dramatically.The relentless violence triggered a fresh wave of civilians streaming across the border into neighboring Turkey. Turkish officials said more than 1,500 Syrians arrived over the past 24 hours, increasing the number of refugees in Turkey to about 51,500.

Rebels control several border crossings into Turkey. At least one of them, Bab al-Salama, near Azaz, is operating normally, with Turkish officials on the other side stamping passports for people passing into Syria.

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