Syrian troops have repelled an attack by jihadists from the Islamic State group on a key military airport in the eastern province of Deir Ezzor, a monitoring group said Sunday.
Elsewhere, rebels fighting to oust President Bashar al-Assad blew up a tunnel near an ancient mosque in the northern city of Aleppo, as loyalists tightened the noose around opposition positions in the same province.
"Troops and pro-regime militia stopped the attack that Islamic State launched on the Deir Ezzor military airport," the Syrian Observatory for Human Rights said, adding both sides suffered heavy losses.
The jihadists had withdrawn to the edges of the base, a day after managing to seize a southeastern part of the complex.
The Observatory said more than 100 jihadists had been killed in fighting for the base since Wednesday when they launched a bid to take the airport.
Pro-regime forces also suffered heavy casualties, with some 59 troops killed, it said.
State news agency SANA said Syrian army units had "repelled an attempt" by IS fighters to attack positions at the base, but provided no further details.
The Deir Ezzor base is a key regime outpost from which warplanes and helicopters mount raids on jihadist positions in several parts of the war-wracked country.
IS fighters control most of Deir Ezzor province, but half of its capital remains in government hands.
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The oil-rich region lies between IS-controlled Raqa province and the border with Iraq, and is a key prize for the jihadist group which declared an Islamic "caliphate" straddling the two countries in June.
On another front, the army secured a fresh advance in Aleppo province, said the Observatory, adding troops killed at least 24 Syrian rebels and jihadists fighting northeast of the country's second city.
"The army... took Breij area northeast of Aleppo city," said Observatory director Rami Abdel Rahman.
The advance meant the army was closing in on rebels in the east of Aleppo city.
"There is a very real threat that the opposition's supply route will be cut off," Abdel Rahman told AFP.
Meanwhile in the heart of Aleppo city, rebels blew up a tunnel near an ancient mosque, claiming to be targeting army positions, said the Observatory.
State television also reported the explosion, and said the rebels had blown up the Sultaniyeh mosque itself.
The Observatory said the mosque was not damaged, but that 12 troops were killed in the blast.
Much of the Old City of Aleppo, home to multiple ancient religious and cultural sites, has been destroyed by more than two years of savage fighting.