As the U.S. military strikes Sunni militants in Iraq, Syrian President Bashar al-Assad's forces have significantly stepped up their own campaign against militant strongholds in Syria, carrying out dozens of airstrikes against the group's headquarters in the past two days. 

While the government in Damascus has long turned a blind eye to the ISIS's expansion in Syria — in some cases even facilitating its offensive against mainstream rebels — the group's rapid march on towns and villages in northern and eastern Syria is now threatening to overturn recent gains by government forces. 

While Islamic State in Iraq and Syria militants have so far concentrated their attacks against the Western-backed fighters seeking to topple Assad, they have in the past month carried out a major onslaught against Syrian army facilities in northeastern Syria, capturing and slaughtering hundreds of Syrian soldiers and pro-government militiamen in the process. 

On Monday, ISIS fighters were closing in on the last government-held army base in the northeastern Raqqa province, the Tabqa air base, prompting at least 16 Syrian government airstrikes in the area in an attempt to halt their advance. 

In the northern city of Aleppo, there is a sense of impending defeat among mainstream rebels as Islamic militants systematically routed them last week in towns and villages only a few kilometres north of the city. An ISIS takeover of rebel-held parts of Aleppo also would be disastrous for Syrian government troops who have been gaining ground in the city in past months. 

"I think they (Syrian government) are finally realizing that their Machiavellian strategy of working with the Islamic State group against the moderates did not work so well, and so they have started to fight it," said Andrew Tabler, a senior fellow at the Washington Institute for Near East Policy. 

But in hitting hard against ISIS, Assad has another motive. His aerial bombardment of militant strongholds in Syria in a way mirrors that of the U.S. military's airstrikes against extremists across the border in Iraq.

SYRIA-CRISIS RAQQA ISIS airstrikes

Syrian government forces struck ISIS positions in and around the eastern Syrian city of Raqqa, including this water plant. (Reuters)

Analysts say Assad's strikes aim at sending a message that he is on the same side as the Americans, reinforcing the Syrian government's longstanding claim that it is a partner in the fight against terrorism and a counterbalance to extremists. That comes after the U.S. itself nearly bombed Syria after it blamed Assad's forces for a chemical weapons attack on rebel-held areas near Damascus last August.  

"Assad would surely love to regain international acceptance via a 'war on terror' and maybe that is his long-term plan, in so far as he has one," Syria analyst Aron Lund said. 

Even while going against the ISIS in Iraq, U.S. officials have shown little appetite for striking at the same militants in Syria. Assad knows that the U.S. administration doesn't have much of a plan for Syria, except to muddle through the mess created by more than three years of civil war. 

Most of all, however, Assad can simply no longer afford to ignore the growing threat of ISIS now that it has started attacking his own forces. 

Since July, following their blitz in Iraq and after they declared a self-styled caliphate straddling the Iraq-Syria border, ISIS
fighters have methodically gone after isolated government bases in northern and eastern Syria, killing and decapitating army commanders and pro-government militiamen. 

'They will stop at nothing'

The attacks started with a devastating onslaught on the al-Shaer gas field in Homs province in which more than 270 Syrian soldiers, security guards and workers were killed. Last month, the jihadis overran the sprawling Division 17 military base in Raqqa province, killing at least 85 soldiers. Two weeks later, ISIS fighters seized the nearby Brigade 93 base after days of heavy fighting. 

They now are closing in on Tabqa air base. Activists on Monday reported intense clashes between government troops and ISIS fighters on the edge of the villages of Ajil and Khazna near Tabqa. The Raqqa Media Center, an activist collective, said ISIS captured four villages near the air base, including Ajil. 

"They will stop at nothing. If things continue the same way it's only a matter of time before ISIS seizes Aleppo," said Abu Thabet, an Aleppo rebel commander. He said the jihadis were now looking to take the rebel stronghold of Marea, to be followed by the Bab al-Salama border crossing with Turkey, which would be a major prize and source of money. 

Meanwhile, a local official and a resident say clashes have broken out between Iraqi security forces and militants on the outskirts of Tikrit.
 
They say the clashes began early Tuesday on the southwestern outskirts of the militant-held city, located north of Baghdad. 
 
Both spoke on condition of anonymity, fearing their safety.
 
The Iraqi military shelled militant positions inside and outside the city, they said. There were no immediate reports of casualties.