By Tom Perry
BEIRUT (Reuters) - Insurgents have seized a historic town in southern Syria from the government, a group monitoring the war said on Wednesday, part of a counter attack rebels say they are waging to stop Damascus reclaiming the border zone near Israel and Jordan.
A Syrian military source said there had been heavy fighting with armed groups on Tuesday night in and around Bosra al-Sham, whose ancient city is listed as a UNESCO world heritage site. "We are investigating the facts in the field," the source said.
An alliance of mainstream rebels who are backed by President Bashar al-Assad's Arab and Western foes said they had taken Bosra and declared the start of a new attack against government forces in another area of Deraa province to the northwest.
Bosra's historic sites include an ancient citadel built around a 2nd century Roman amphitheatre. It was the capital of the Roman province of Arabia.
Syria's southwestern corner is of strategic importance due to its proximity to Damascus and neighboring states Israel and Jordan. It is also the last significant foothold of mainstream rebels, who have mostly been crushed elsewhere in Syria by government forces or jihadist groups.
An offensive launched by Damascus in early February made early gains but its territorial advance then slowed.
The mainstream rebels say they have received more military support from Assad's foreign foes since the start of the push by the Syrian army and allied militia including the Lebanese group Hezbollah.
Support to the Southern Front groups has been channeled via Jordan, a staunch ally of the United States.
Washington maintains its view that Assad has lost legitimacy and must leave power even as its focus has switched to fighting the Islamic State jihadist group that has seized wide areas of northern and eastern Syria, as well as chunks of Iraq.
The Syrian Observatory for Human Rights, the monitoring group, said 21 insurgents had been killed in four days of fighting for Bosra, some 20 km (12 miles) north of the border with Jordan. The Syrian army on Monday said it had killed several insurgent leaders there.
The spokesman for the Southern Front alliance said 85 percent of the fighters who took part in the attack were from mainstream rebel groups, with the rest from Islamist factions.
The Nusra Front, al Qaeda's wing in Syria, had not taken part, said spokesman Issam al-Rayyes. "We liberated the whole city, even the old castle and old city," he said.
The Southern Front had launched the attack because the army had been mobilizing pro-government militia in Bosra for a new stage of its offensive in the south, he added.
The new attack declared by the Southern Front on Wednesday was aimed at taking a place called Jadieh some 80 km (50 miles) to the northwest of Bosra.
"Our strategy was not to hold on to territory, but to attack in areas where the regime was not expecting it," Rayyes said.
"We were able to stop the regime's progress, and we have started to attack. We have started to liberate additional areas," he said.
(Additional reporting by Suleiman al-Khalidi in Amman; Writing by Tom Perry; Editing by Alison Williams, Andrew Heavens and Crispian Balmer)