Syrian opposition fighters have seized the largest remaining military base in Idlib in one of their most significant victories in the province since taking Idlib city in March, reports say.

Abu Yazid, a spokesman for Ahrar al-Sham, one of Syria's most powerful armed groups, told Al Jazeera the fighters took over the entire town of al-Mastouma and its army base after days of heavy clashes with security forces.

Ahrar al-Sham is part of the Fattah Army, a coalition that controls most of Idlib province, in Syria's northwest.

Yazid also said that a subsequent offensive in the northern town of Ariha, one of the last government bastions in Idlib, is forcing Syrian troops to withdraw.

"The military has begun withdrawing from Ariha," he said.

He said at least 50 troops had been killed in recent fighting in Idlib.

The Syrian Observatory for Human Rights, a UK-based monitoring group, and numerous activist groups on the ground in Syria have confirmed the takeover of entire al-Mastouma.

Syrian state television reported that army units at the base were moved to Ariha to reinforce defence lines there.

Ahrar al-Sham's advances come almost two months after the Fattah Army seized Idlib city, which is strategically located near the main highway connecting the the country's second city Aleppo and the capital Damascus.

Heavy fighting has also continued in the Idlib city of Jisr al-Shughur, where government forces are bombarding rebel-held areas in an attempt to rescue many soldiers reportedly besieged in a hospital.

Praise for Iran

Against this backdrop of military reverses, Syrian President Bashar al-Assad has praised Iran, the main foreign source of military, political and economic support for his government, as a "key pillar".

Tuesday's statement came during Assad's meetings with Ali Akbar Velayati, a foreign affairs adviser to Iran's supreme leader, who was the third Iranian official to visit Damascus in less than a week.

"The support given by Iran to the Syrian people constitutes a key pillar in the battle against terrorism," state news agency SANA quoted Assad as saying.

Velayati's visit came days after the two countries struck a series of major economic deals in a wide range of sectors, including electrical, medical and oil industries, according to SANA.

Last Thursday, Alaedin Boroujerdi, head of the Iranian parliament's national security and foreign policy, visited Damascus and said that Iran's support for the Assad government was "firm and eternal".