* Assad trying to mass forces on edges of Aleppo province
* Third day of Assad air strikes on neighbouring Saraqeb
* Explosion in central Damascus kills one, wounds several
By Erika Solomon
BEIRUT, July 20 (Reuters) - Syrian rebels advanced on the northern town of Khan al-Assal on Saturday, activists said, and appeared close to seizing one of the last towns in western part of Aleppo province still held by President Bashar al-Assad's forces.
The army has been attempting a slow build-up of troops around the province in order to retake Aleppo city, once Syria's biggest commercial hub. So far Assad's push has been dogged by rebel counter-attacks, although a string of government victories elsewhere in Syria has shifted the battlefield tide in his favour after more than two years of bloodshed.
Elsewhere in northern Syria, Assad's forces launched a third day of heavy air strikes on the town of Saraqeb in Idlib province. The Syrian Observatory for Human Rights said that by late afternoon, jets had already flown 12 raids over the town. Initial counts said five had been killed on Saturday, three of them children.
Neighbouring Idlib and Aleppo provinces are regions in northern Syria that have become a stronghold for rebels fighting to end four decades of rule by the Assad family.
It was unclear why Saraqeb has been so heavily targeted, but some local activists suggested this was retaliation for a rebel capture of a nearby army checkpoint.
Others posited that the army was trying to hammer areas near a critical road leading to Aleppo in order to distract the rebels and be able to bring in supplies to its forces.
Rebels have been blockading government-held areas in Aleppo city, Syria's largest urban centre. Aleppo has been mired in a bloody stalemate since rebels launched an offensive in the province last year.
Street protests-turned-insurgency in Syria have killed more than 100,000 people since March 2011, according to the Observatory, and have destroyed ancient buildings and artefacts across the major Arab state.
Assad's forces have been on the offensive since last month when the army, backed by militants from the powerful Lebanese guerrilla group Hezbollah, recaptured a border town near Lebanon straddling supply lines between Damascus and Syria's seacoast.
Hezbollah-backed Syrian forces have been bearing down hard on rebel pockets around Damascus and other parts of the central province of Homs, part of a strategy to cement control of a belt of territory between the capital and his stronghold on the Mediterranean coast, before focusing on other regions.
BLAST IN DAMASCUS
While his forces seem to be nearing that goal around Homs and on the outskirts of the capital, they have made little progress around the northern and southern borders where rebels have a foothold.
Around Damascus, the army has made gains in the countryside but has faced an uptick of rebel bomb attacks inside the capital in recent weeks. The rebels often use car bombs and other explosives to hit at areas where they have a weaker presence.
An improvised explosive device hit a central neighbourhood of Damascus on Saturday, killing one person and wounding several others, according to the Britain-based Observatory, which has a network of activists across SYria.
Activists also said that several rockets hit the central Abbassid Square of the city, damaging several buildings.
In recent weeks the rebels have been making slow but persistent progress in Deraa province in the south thanks to a fresh but intermittent influx of weapons coming in from neighbouring Jordan.
In Aleppo, near the Turkish border, hardline Islamist rebels - some of them linked to al Qaeda - appear to be leading the fight to seize Khan al-Assal, according to activists in the area who asked not to be named.
Western powers like the United States are alarmed about the rising power of radical Islamist groups, particularly since Washington has pledged to offer military support to Assad's opponents. No military aid has been given yet due to political deadlock over the Islamist issue in the U.S. Congress.
"Perhaps the Islamists are trying to stay out of the spotlight. They've been regrouping and naming themselves with numbers, things like 'the 9th Division' and so on, but these are the same Islamist radical groups like the Islamic State of Iraq and Sham or the Islamic Front to Liberate Syria," one opposition activist said, declining to be named.
Assad's forces have responded to the rebel advance on Khan al-Assal with a string of air raids in the area that set alight swathes of nearby farmland, according to the Observatory.
It counted at least 12 soldiers and four rebels killed in the latest fighting. (Editing by Mark Heinrich)
Our Standards: The Thomson Reuters Trust Principles.