BEIRUT - Heavy fighting in southern Syria has killed scores of pro-government and insurgent fighters in the past week, a group monitoring Syria’s war said on Sunday, forecasting even fiercer violence as the weather clears.

Syria’s army and allied combatants from Lebanon’s Hezbollah launched a large-scale offensive in the region last week against insurgent groups, including al Qaeda’s Syria wing Nusra Front and non-militant rebels. Southern Syria is one of the last areas where mainstream rebels opposing President Bashar al-Assad have a foothold. They have lost ground to hardline Islamist militants in the four-year conflict.

More than 50 rebels have been killed in the fighting, the head of the Britain-based Syrian Observatory for Human Rights said. Rami Abdulrahman said 43 members of the Syrian army and allied groups had also died, including 12 officers. ‘Now the weather is better there will be Syrian air strikes. With the air strikes they will move forward,’ he said. State news agency SANA said Syrian forces had inflicted ‘heavy losses’ on ‘terrorist groups’ in Deraa on Sunday after killing many enemy fighters and destroying their vehicles the previous day.

Syrian officials were not immediately available for comment on Sunday. State media and Hezbollah’s al-Manar channel have carried regular reports on the fighting in the south. Abdulrahman, who tracks the war using sources on the ground, said around 5,000 pro-government troops were taking part in the offensive, which aims to take a triangle of rebel-held land from rural areas southwest of Damascus to Deraa city to Quneitra.

Sources on both sides of the battlefront have said the offensive aims to shield Damascus, the capital, which is a short drive to the north. The insurgents made significant gains in the south in recent months, taking several army bases. ‘The situation remains hit-and-run between us and regime forces,’ said Abu Gaiath, a spokesman for the rebel Alwiyat Seif al-Sham group. Its fighters are part of the ‘Southern Front’ rebel alliance that has had support from states opposed to Assad.

Speaking via the Internet from inside Syria, he said fighting had calmed in the past two days but the military was aiming to encircle a village northeast of Quneitra and had captured towns and villages south of Damascus. The Observatory’s Abdulrahman said 10 fighters on the government side had been executed after being accused of passing information to the enemy.

He also said Nusra Front fighters had been killed in battle but exact numbers were not known. Winter weather had curtailed fighting in the past few days and prevented air strikes, one of the army’s most potent weapons against insurgents. Abdulrahman said the army and allied groups planned to involve 10,000 fighters in the offensive. That battle marked the first time in months that fighters from the Kurdish People’s Protection Units (YPG) have expanded their attacks to the edge of Raqa province.

Four YPG fighters were killed in those clashes, with the Kurdish forces capturing the strategic Tal Baghdaq hilltop, the Observatory said. The other militant deaths came in clashes to the west and south of Kobane, the key border town that Kurdish forces recaptured on January 26. IS fighters began advancing on the town last September, and at one point looked poised to overrun it, cementing their grip on a long stretch of the Syria-Turkey border.

But YPG ground forces backed by US-led air strikes were able to repel the IS fighters. Since the town’s recapture, YPG forces have been expanding their control from Kobane into the surrounding countryside, where they have largely encountered little resistance from the militants.

But the Observatory said the fighting around Tel Baghdaq on the Aleppo-Raqa border was some of the heaviest in the region since Kobane fell. The group said the US-led coalition fighting IS had also carried out air strikes in the region on Sunday, but there was no initial information on casualties in the raids. The IS group has declared an Islamic ‘caliphate’ in territory it controls in Syria and Iraq, with the provincial capital of Raqa province serving as its effective capital. More than 210,000 people have been killed in Syria since the country’s conflict began with anti-government demonstrations in March 2011.