Beirut, Reuters—Intense clashes in southern Syria have 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 the Al-Nusra Front and non-jihadist rebels.

The battle is significant because it is one of the last areas where mainstream rebels opposing President Bashar Al-Assad have a foothold. Such groups have lost ground to hard-line 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 airstrikes. With the airstrikes they will move forward,” he said.

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 Dera’a city to Quneitra.

Sources on both sides of the battlefront have said the offensive aims to shield Damascus, the capital a short drive to the north. The insurgents had made significant gains in the south in recent months, taking several army bases.

Syrian state media and Hezbollah’s Al-Manar channel reported on the battles in the south during the week, saying the army had been reclaiming territory from “terrorists.” State media said a number of enemy fighters had been killed. Syrian officials were not immediately available for comment on Sunday.

“The situation remains hit-and-run between us and regime forces,” said Abu Gaiath, 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 Al-Nusra Front fighters had been killed in battles but exact numbers were not known.

Winter weather had limited fighting in the past few days and prevented airstrikes, 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.