BEIRUT — Rebel groups fighting the Syrian government have seized control of a southern town near the border with Jordan after four days of battles, the Syrian Observatory for Human rights reported on Wednesday. A Syrian military source said there had been heavy fighting with armed groups in and around the town of Bosra Al-Sham on Tuesday night. “We are investigating the facts in the field,” the source said. The ancient city of Bosra is listed as a UNESCO world heritage site. The Syrian army and allied forces including the Lebanese group Hezbollah launched a big offensive against rebel groups in Syria's southwestern corner early last month. It is an area of strategic importance due to its proximity to Damascus and neighboring states Israel and Jordan. The Observatory said 21 insurgents had been killed in the four days of battles. The Syrian military on Monday said it had killed several insurgent leaders during the fighting. Bosra Al-Sham is around 20 km (12 miles) north of the Jordanian border in the province of Deraa. Bosra's historic sites include an ancient citadel built around a 2nd century Roman amphitheater. Bosra was once the capital of the Roman province of Arabia, according to the UNESCO website. More than 220,000 people have been killed since the conflict in Syria began in March 2011. The crisis started with largely peaceful protests calling for reforms and escalated into civil war following a brutal government crackdown. The Observatory said Kurdish gunmen killed at least 28 IS fighters in a special operation in the northern province of Raqqa. It added that members of the main Kurdish militia, the People's Protection Units, or YPG, were able to capture the bodies of the dead militants. YPG fighters have been battling the IS group for months, capturing scores of villages. The Kurds blamed IS for two bombings last week that killed 49 people celebrating the Kurdish New Year in the northeastern Syrian city of Hassakeh. Also on Tuesday, Russian Ambassador to the United Nations Vitaly Churkin said the next round of Syria talks in Moscow will be between April 6 and 9. The first round of Moscow peace talks was held in January and was boycotted by most rebel groups as well as the main Western-backed Syrian National Coalition. Russia is a key ally of Assad's government. "We think it's time for a breakthrough in a political settlement," Churkin told reporters. — Agencies