Amid heavy fighting in Syria's two main cities, Israel yesterday issued a thinly veiled warning that military action would be used if necessary to prevent the Damascus regime's chemical weapons from falling into the hands of the Lebanese militant group Hezbollah.

During a third day of fierce street battles in Aleppo, a leader of Syrian rebel forces said they were fighting to "liberate" the country's second city, long seen as a stronghold of President Bashar al-Assad. And in Damascus, government troops backed by helicopter gunships appeared to be regaining some ground in their effort to win back neighbourhoods seized by the rebels over the last few days.

Israel's Defence Minister, Ehud Barak, reaffirmed that its military and intelligence services were closely tracking the possibility of what he called "advanced weapons systems" being acquired by Syria's Hezbollah allies. He warned that Israel "cannot accept a situation" whereby such systems are transferred by Syria to the Shia guerilla group. Mr Barak said it was "inappropriate to say when we would act, how we would act – indeed if we would act".

His comments were echoed by the Prime Minister, Benjamin Netanyahu, who told Fox News that Israel would "have to act" if there was any risk of weapons falling into the hands of militant groups.

Hezbollah tops Israel's concerns about the fallout from a Syrian conflict that has now widened to include the capital and the second city. And some officials have canvassed the possibility that any convoys suspected of ferrying chemical stocks to the Lebanese militants – whom Israel has also blamed for the suicide bombing on the Bulgarian Black Sea coast which killed five of its citizens last week – could be targeted.

But Israeli officials have said they are also worried that, if its hostile northern neighbour fragments, sophisticated weaponry could find its way into the hands of militant Islamist groups, including al-Qa'ida; or even that, in the last throes of power, the regime might decide to turn its weapons on Israel itself. All three possibilities were discussed in a series of meetings with Israeli officials held by Tom Donilon, President Barack Obama's national security adviser, earlier this month.

And they are also likely to be on the agenda during a visit by the US Defence Secretary, Leon Panetta, at the end of the month. While sharing Israeli concerns , especially after warnings by Syrian defectors that it could use chemical weapons against opposition forces, the US has reportedly shown no enthusiasm for Israeli attacks on Syrian weapons facilities.

Some prominent Israeli analysts have also been highly sceptical about such a military intervention, particularly a unilateral one, on the grounds that technically complex and air-delivered weapons systems could not easily be exploited by non-state militant groups. Defence official Amos Gilad said regime forces were "protecting these arsenals as best they can". Amos Harel, Haaretz's military commentator, suggested yesterday that one possibility was an Israeli attack on Hezbollah in Lebanon.

On the ground in Syria, fierce battles continue, with residents and opposition activists telling Reuters that rebels had been driven from Mezzeh, the diplomatic district of Damascus.

They said that more than 1,000 government troops and allied militiamen poured into the area, backed by armoured vehicles, tanks and bulldozers.

The fierce fight in Damascus neighbourhoods, around the regime intelligence headquarters in Aleppo, and in the eastern town of Deir al-Zour, apparently reflected on the regime's desire to avenge the deaths of four members of Mr Assad's high command last week. Syrian forces regained one of two border crossings that had been taken by rebels on the frontier with Iraq, according to Iraqi officials.

But the rebels said they had captured a third border crossing with Turkey, Bab al-Salam, north of Aleppo. And last night they claimed to have secured an infantry school 10 miles north of Aleppo. "This is of big strategic and symbolic importance. The school has ammunition depots and armoured formations and it protects the northern gate to Aleppo," Brigadier General Mustafa al-Sheikh told Reuters.