* Base is strategic gain on vital southern highway
* Monitoring group records bloody battle near Israel border
* Southern areas grow in importance for supplying capital
Rebels seized an air defence base near Syria's strategic southern international highway on Saturday, activists said, a gain that could bolster opposition fighters trying to secure supply routes to the capital Damascus.
Syria's southern provinces bordering Jordan and Israel have become an increasingly significant battleground as the embattled capital comes into play, with President Bashar al-Assad's forces and his loyalist militias hitting back hard to prevent rebel advances.
The Britain-based Syrian Observatory for Human Rights, which has a network of sources among activists across Syria, said among those killed was the base commander after a two-week battle for the site in Deraa, a province bordering Jordan where initially peaceful protests against Assad set off Syria's two-year uprising.
"Fighters from the Nusra Front, Yarmouk Martyrs Brigade, and other battalions seized control of the 38th division air defence base, near the town of Saida on the Damascus-Amman highway, after 16 days of fierce clashes," the Observatory said.
The Nusra Front is an Islamist militant group suspected of links to al Qaeda and blacklisted by the United States as a "terrorist group". Its forces, which include foreign fighters, have come to increasing prominence in the revolt.
A video uploaded by the Observatory, showed what it said was the body of General Mahmoud Darwish, commander at the base, lying in a pool of blood in a bathroom.
It was not immediately possible to verify the pictures or activist report. The Syrian government has severely restricted access to Syria for foreign journalists and international aid groups.
CLASHES NEAR ISRAELI BORDER
Clashes have also raged near the border with Israel, which is growing concerned that Islamist rebels may be emboldened to end the quiet on the Golan front maintained by Assad and his father before him during their four decade rule of Syria.
Israel captured the Golan plateau from Syria in the 1967 Middle East war.
Rebels last Wednesday overran at least three towns near the Israeli-Syrian disengagement line but appeared to lose ground after militias loyal to Assad pushed them back in a fierce battle.
Rami Abdelrahman, head of the Observatory, said he recorded one of the heaviest single death tolls for rebels in the fighting near the Israeli border.
"We have 35 fighters recorded by name and at least 20 more missing, this is a huge number to be lost in a single battle and shows how hard they (Assad's forces) are fighting for these areas," Abdelrahman told Reuters.
He said the pro-government militias involved were made up of fighters from Syria's Druze minority, a sect which for some time had stayed on the sidelines of the conflict but now increasingly appears to be swinging its weight behind Assad.
Their involvement could increase sectarian bloodshed in Syria, already wracked by tensions between the Sunni Muslim majority that has led the uprising and the Alawite minority to which Assad belongs.
Tit-for-tat kidnappings and killings between the sects have become common as the uprising spiralled into a bloody civil war that has killed more than 70,000 people.
Alawite opposition campaigners, increasingly concerned for the fate of their community as the fighting continues, will meet this weekend to discuss options for supporting a democratic alternative to Assad's rule and try to distance themselves from security forces attempts to crush the revolt.