Syria's army broke a more than year-long jihadist siege of a military air base in the country's north Tuesday, scoring its first major breakthrough since Russia's air campaign began.
But the government advance came as at least 22 people were killed in one of the bloodiest mortar attacks yet on the regime's coastal bastion of Latakia.
Troops, backed by pro-government militia, broke through the Islamic State group's siege of the Kweyris military airport in northern Aleppo province, a photographer working with AFP said.
A group of soldiers penetrated IS lines west of the airport and reached government troops inside the base, firing into the air in celebration.
State television also reported the breakthrough and broadcast live from outside the airport.
It said a "large number of IS terrorists" were killed but provided no details.
SANA, the official news agency, said President Bashar al-Assad called the commanders of both the airport and the soldiers who made the breakthrough to congratulate them.
IS surrounded Kweyris in spring 2014, tightening a siege that rebel groups began in April 2013.
Loyalists and IS jihadists remained locked in fierce clashes to the airport's north, east, and west Tuesday evening, said Rami Abdel Rahman, head of the Britain-based monitor the Syrian Observatory for Human Rights.
IS militants were still present in other areas around the airport and further east in Aleppo province, he told AFP.
The offensive to break the siege began at the end of September, with support from Russian warplanes, Iranian troops, fighters from Lebanese Shiite militia Hezbollah and the pro-government National Defence Forces militia, Abdel Rahman said.
If the base is completely secured, it could be used by Russian planes currently flying out of Hmeimim in Latakia province, he said.
"In particular, it will help their efforts to retake all of Aleppo city," which is divided between rebels and regime forces, Abdel Rahman said.
Russia's air force has been conducting strikes across Syria since September 30 against what it calls "terrorist targets".
The air campaign has backed ground offensives in several provinces, including Aleppo, Hama and Homs in the centre, and Latakia in the west.
But the regime has struggled to advance against opposition forces, often briefly seizing a town before losing it again.
By the first week of November, Islamist groups and rebels had rolled back all of the army's gains in Hama.
In Aleppo, troops backed by Russian air power have edged into territory controlled by rebels and IS south and southeast of the provincial capital, but securing Kweyris would mark the first major victory in the area.
The breakthrough came only hours after one of the deadliest attacks in the conflict in the regime's seaside stronghold in Latakia.
At least 22 people were killed and 62 wounded in mortar fire on eastern neighbourhoods of Latakia city, state television reported.
Latakia lies in the heartland of the minority Alawite sect to which Syria's ruling clan belongs, and has been largely spared attacks during four and a half years of civil war.
A security source said the attack took place near Latakia's Tishreen University, where many students were gathered.
Abir Selman, a 24-year-old literature student, said she was waiting for the bus when the mortars struck.
"I saw blood everywhere and people running in every direction," she told AFP. "I passed by a corpse that had nothing left except for its legs."
Meanwhile, one person was killed and five wounded in a mortar attack on residential areas of Damascus, the state broadcaster said.
The Observatory said four people, including a child, were also killed in government rocket fire on the flashpoint town of Douma, east of Damascus.
Douma is in the opposition bastion of Eastern Ghouta, which is regularly bombarded by regime forces.
As well as the Russian strikes, IS is being targeted by a US-led coalition including France, which said Tuesday it had stepped up its bombing campaign against IS's oil infrastructure with new raids in eastern Syria.
Speaking on the sidelines of a conference in Senegal, Defence Minister Jean-Yves Le Drian said France "struck again twice last night in the Deir Ezzor region, firstly on an oil distribution station and secondly on a gas separation plant".
In Washington, Air Force General Herbert Carlisle said communications between US and Russian military officials have vastly improved since the start of Russia's bombing campaign.
"They talk twice a day, there's a hotline," Carlisle said.
"Everybody is interested in safety of flight and everybody is interested in not getting into dangerous situations or miscalculations... If you look at what's going on, it has improved since they (the Russians) came."