The Islamic State group gained ground around a key eastern Syrian city on Sunday, a monitor said, despite a heavy air strikes by government warplanes.

The jihadists pressed their brutal assault on Deir Ezzor for the second day, seizing territory near an air base on the city's edges, according to the Syrian Observatory for Human Rights.

"IS seized several hilltops that overlook the airport," and clashes gripped the area on Sunday, said Observatory head Rami Abdel Rahman.

The advance came "despite more than 120 air strikes carried out by regime forces on jihadist positions since Saturday morning, in addition to heavy artillery fire," he said.

Around 200,000 people live in Deir Ezzor city, which has been besieged by IS since early 2015 and is the capital of the oil-rich province of the same name.

IS already controls more than half the city, but it has sought to capture the remaining government-held territory, including the military airport.

It launched one of its fiercest assaults yet on Saturday, unleashing a wave of tunnel bombs and suicide attackers.

At least 12 government fighters and 20 IS jihadists were killed.

A military source in Deir Ezzor told AFP that IS was "amassing its forces to attack Deir Ezzor and breach government lines" in order to cut off regime access to the base.

The extremist Sunni Muslim group has lost swathes of territory in northern Syria to Kurdish fighters as well as a Turkish-backed rebel alliance, but it remains on the offensive in other parts of the country.

In addition to its push in Deir Ezzor, IS recently recaptured Palmyra in central Syria from government forces.

The jihadist faction is excluded from a nationwide ceasefire in Syria, in place since December 30 but increasingly strained by fighting.

The truce is meant to pave the way for peace talks later this month in the Kazakh capital Astana.

Anti-IS fight going as fast as possible: Pentagon official
Washington (AFP) Jan 13, 2017 - The US-led coalition attacking the Islamic State group in Iraq and Syria would struggle to dramatically quicken the campaign, a top Pentagon official said Friday, amid mounting pressure to accelerate the fight.

President-elect Donald Trump has repeatedly said he wants to "bomb the shit" out of IS, and on Thursday his pick for defense secretary, James Mattis, said operations could be intensified -- especially in the push toward the jihadists' stronghold of Raqa in Syria.

But Elissa Slotkin, the acting assistant secretary of defense for international security affairs, said the pace of the Raqa campaign can only go as fast as the coalition-backed local forces on the ground.

Commanders "have a plan that I believe is pushing to the limit what we can do on intensifying that campaign," said Slotkin, a political appointee who will not be working under Trump.

In Syria, the coalition is providing weapons, training and air support to Kurdish and Arab forces as they work to push IS from Syria.

A similar tactic is underway in Iraq, with the coalition backing Kurdish peshmerga and Iraqi security forces as they fight for Mosul.

Slotkin said anti-IS operations in Syria were creating a "snowball effect," with each new victory generating more recruits willing to fight IS.

She estimated the coalition-backed forces, known as the Syrian Democratic Forces, now numbered about 50,000.

Speaking more broadly of the anti-IS effort, Slotkin said she had a "hard time" seeing how the campaign could proceed much faster, and did not see what additional targets could be attacked that weren't already on the military's radar.

"I would have some questions about what exactly are you striking if you just launched a big campaign," she said.

The coalition has been making steady gains against IS, with the jihadists having lost control of many of the main cities they had held in Iraq in Syria.

Outgoing Pentagon chief Ashton Carter said in an interview Thursday that IS leader Abu Bakr al-Baghdadi's "days are numbered."