BEIRUT (Reuters) - Syrian rebels said they were struggling to make headway against a barrage of government jet and artillery attacks in their latest attempt to take control of the country's largest city Aleppo after weeks of deadlock.
On the second day of an offensive they had billed as a "decisive battle", rebels also threatened to start fighting local Kurdish militants - a move which would further complicate a war that has already spilled over Syria's borders.
Fighters reached by telephone said they had been locked in hours of fierce combat in several neighborhoods of Aleppo, Syria's commercial hub, on Friday.
Rebels armed with machineguns and homemade rockets said they faced a difficult task against a better-equipped enemy.
"We reached the middle of Suleiman al-Halabi and liberated some neighborhoods so I am still optimistic. But I'm worried about our organization. We can't force the regime out. At best, I think we can advance some of our positions," one fighter said, requesting anonymity.
Other rebels told Reuters one of their units had been surrounded. One fighter said some insurgent battalions were pulling out of the front line or had never joined the battle.
The 18-month-old uprising against President Bashar al-Assad began as peaceful protests but has descended into a civil war. More than 30,000 people have been killed, say activists.
Syria's government says it is fighting Islamist hardliners, adding that thousands of Arab and foreign fighters have entered the country from Turkey.
World powers have been meeting at the United Nations this week but are divided over the crisis.
Russia, China and Iran back Assad and oppose any UN sanctions on Syria's leader.
Western countries and Arab states supporting the opposition remain unwilling to take forceful action, despite Qatar's calls for Arab intervention. Some western diplomats say they have been frustrated by what they see as a lack of clear command structure and coordination among the rebels.
One group of countries sympathetic to Syria's opposition is planning to hold another "Friends of Syria" meeting, but there was little prospect of that resulting in action against Assad.
"I just expect ideas to be presented. There will be no concrete plans," Arab League Secretary General Nabil Elaraby told Reuters at the conference in New York.
Diplomats on Friday said that Carla del Ponte, the International Criminal Court's former chief prosecutor, will be named to join a UN investigation into abuses in Syria. Del Ponte was known for doggedly pursuing former Yugoslav President Slobodan Milosevic at the ICC. [ID:nL5E8KS58C]
Human rights investigators say state forces and some rebel groups have committed war crimes in Syria.
Inside Syria, neither side seems ready to put down its arms. Assad's forces have pounded rebel-held areas across the country, and clashes erupt daily. Yet both sides appear incapable of striking a decisive military blow.
A main international concern has been the security of Syria's chemical weapons sites. But U.S. Defense Secretary Leon Panetta cited U.S. intelligence on Friday suggesting that the chemicals remained secure.
The rebels appear to be improving their reach. A bomb attack on Wednesday wrecked the army's command headquarters in the heart of Damascus, though no major officers were killed.
The rebels also threatened to confront locally-based Kurdish militant groups who they said they suspected of supporting Assad.
They said the groups were linked to Kurdish Worker's Party (PKK) which has been fighting for autonomy in neighboring Turkey.
One rebel leader issued a warning to the Kurds through the Facebook page for the Tawheed Brigade, the largest rebel unit in Aleppo.
"Tawheed Brigade leader Abdelqadir al-Saleh made a final request by phone to the PKK gangs, to drop their weapons immediately and not drag themselves into a losing battle that is not their fight," it said.
"Whoever carries arms in the face of the opposition battalions will find themselves under fire."
In Aleppo's Kurdish neighborhood of Sheikh Maqsoud, rebels said they had captured at least eight men from the shabbiha - as the pro-Assad militias are known. Some of the captives were killed, they said.
It was unclear if the victims were Kurds, a stateless ethnic group who stretch over much of the region and have so far been split over their support for the uprising.
Reporting by Erika Solomon and Mariam Karouny; Editing by Andrew Heavens