Gun battles rocked the streets of Aleppo on Thursdayas rebel groups tried to push from their stronghold in the south of the city. Meanwhile, diplomatic hopes for a resolution to the crisis were crushed by the resignation of the UN's envoy to Syria, Kofi Annan.

For a second day, rebel groups made use of tanks they had captured from a regime base, shelling an airfield in the north of the city and attacking regime positions, forcing thousands more to join a mass exodus of refugees from parts of Syria's second city.

Abu Hamza, a Free Syrian Army colonel from the Jebel al-Zawiya district south of Idlib, told the Guardian that neither the FSA nor local communities could provide shelter or food for the thousands of displaced civilians being forced to sleep in fields or on the streets of towns and villages.

More than 250,000 refugees are believed to have fled Aleppo in the past fortnight, with large parts of the city of 2.5 million people now empty.

"We can't feed them," he said. "We need help. We don't even have food for our own families, or for ourselves. We cannot survive for much longer under these conditions. We are talking a few weeks."

In southern Turkey, meanwhile, the general formerly responsible for Syria's chemical warfare division said US and Turkish intelligence officers had questioned him about the weapons' location and whether the regime would use them.

Adnan Silou, who fled to Turkey nearly two months ago, said he told his interrogators that the stockpiles of chemicals remained secured, but that regime leaders would likely deploy them if they were cornered. "I am sure about this," he said. "They were a weapon of last resort and what will happen when that day comes."

Silou, who retired from the Syrian military's most controversial unit in late 2008, said he had been consulted by still serving officers throughout the past three years and was able to inspect an inventory of the weapons 10 days before fleeing.

"Every one of the stockpiles was intact, although it appeared that some had been moved," he said. "Not even a centimetre had disappeared from the supplies as I knew them three years ago."

Syria's chemical weapons included Sarin, mustard and nerve gas, which could be deployed via artillery shells, rockets, or aircraft, Silou said. He said making them combat-ready was a difficult process, requiring components to be brought together from various locations across the country.

He identified the main chemical depots as being 10km east of Damascus and 10km south of Homs. "They were called units 417 and 418 and they are heavily protected," he said. Chemicals have also been stored in the eastern desert city of Deir el-Zour.

"All of these things I told the Americans and the Turks when they took me to Ankara," he said. "They wanted to know everything. I told them that only the president could give the order to weaponise them. It would have to be Assad."

With state authority in Syria steadily eroding through defections and sanctions, fears have been raised that the chemical weapons may be used by the embattled regime, or fall into the hands of terror groups.

Syria's foreign ministry spokesman Jihad Makdissi last month added to growing international concerns by warning that the weapons might be used against "foreign aggressors". "Any stock of WMD or unconventional weapons that the Syrian Army possesses will never, never be used against the Syrian people or civilians during this crisis, under any circumstances," he said. "These weapons are made to be used strictly and only in the event of external aggression against the Syrian Arab Republic."

Since then, regional states, including Saudi Arabia, Turkey and Israel, have sharply increased efforts to monitor the location of stockpiles. Israel has suggested it would be prepared to send its military to Syria to safeguard the weapons if the regime fell.

The intractable nature of the conflict was brought into focuson Thursday when Kofi Annan, the United Nations special envoy to Damascus, announced he would not extend his term when it expires at the end of the month.

The diplomat tabled a peace plan in April that had been the linchpin of diplomatic efforts to stop a slide into full-blown civil war. However, none of its main points had taken root, despite three rounds of shuttle diplomacy to Damascus, pleas to both the regime and rebel groups to negotiate, and dire warnings about the failure to find a compromise.

The end of Annan's diplomacy comes amid continuing gridlock at the UN security council, where three attempts by the US and European states to shift Russia and China from their resolute support for the Assad regime have failed.