BEIRUT – 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, which has been 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. But rebels 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 main rebel stronghold of Salaheddine.

“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 U.N. official Lakhdar Brahimi has emerged as a strong candidate to replace Kofi Annan as 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 in the conflict.

Also Friday, the U.S. 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 close ties to Hezbollah — and to the group’s patrons in Iran — mean that 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.

The regime has been trying to drive rebels out of Aleppo, Syria’s largest city, for more than two weeks. The state news agency claimed Wednesday that Assad’s forces had regained control of the Salaheddine neighborhood, the main rebel area in Aleppo. But activists said rebels were still putting up a fight there on Friday.