The Syrian army and allied forces have broken ISIL's siege on Deir Az Zor military airport, state media said on Saturday, hours after US-backed fighters announced a separate push against the armed group in the oil-rich eastern province.

State news agency SANA said the breach came "after the forces advancing from the cemetery southwest of the city linked up with the forces holding the air base".

The latest advance of the Russia-backed forces came days after they ended another siege on residential districts of the city as they seek to remove the Islamic State of Iraq and the Levant (ISIL, known as ISIS) from its last major stronghold in Syria.

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Earlier on Saturday, the Syrian Democratic Forces (SDF), a coalition of Kurdish and Arab fighters, said they had also launched an anti-ISIL offensive from the north of Deir Az Zor city, aiming to clear the group from territory east of the Euphrates River.

The operation brought them into a race with the government forces marching in the same direction.

Ahmad Abu Khawlah, head of SDF's Deir Az Zor Military Council, said his forces had begun fighting to push ISIL out of the remaining territory in Hasakah province and further south in Deir Az Zor in eastern Syria.

The announcement came as Syrian government forces, backed by Russian air power and Hezbollah fighters, closed in from the south of Deir Az Zor city in a separate military campaign against ISIL.

The duelling battles for Deir Az Zor highlight the importance of the key eastern province, which has become the latest epicentre of the international war against ISIL, raising concerns of an eventual clash between the two sides.

Al Jazeera's Mohammed Jamjoom, reporting from Beirut, the capital of neighbouring Syria, described the double offensive as "a race against the clock to see which of these two competing forces can have dominance in this oil-rich area".

He added: "Whoever is going to get more of this province is going to control more of the oil, and this really sets up what has been a regional rivalry to see who can take over more territory in Syria since the beginning of this war."

Battle for Raqqa

The Syrian conflict, which started as a popular uprising against President Bashar al-Assad in 2011, has drawn the military involvement of several world powers. Peace talks have repeatedly failed to bring an end to the war.

The SDF's new push comes as its fighters are still involved in a major offensive to capture Raqqa from ISIL.

Three months into the battle, they have taken around 60 percent of the city, and much more difficult urban fighting still lies ahead.

Earlier this week, Syrian troops and their allies reached Deir Az Zor, breaking a nearly three-year-old ISIL siege on government-held parts of the city in a major breakthrough in their offensive against the group.

In a victory statement, the Syrian military said Deir Az Zor will be used as a launching pad to recapture the remaining ISIL-held areas along the border with Iraq.

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The troops' arrival to Deir Az Zor city brings Syrian forces and their allies a step closer to controlling the oil-rich eastern province and its capital bordering Iraq. The region has some of Syria's largest oil fields, whose revenue is vital to the state's dried coffers.

Syria's military command announced on Saturday they had removed ISIL fighters from the province's Taym oilfield, southwest of Deir Az Zor city, on the other side of the Euphrates.

They also seized part of a main highway running from Deir Az Zor downstream to the city of al-Mayadeen, to which many ISIL fighters have retreated, said the Syrian Observatory for Human Rights, a UK-based monitor. said.

That advance would help block potential ISIL reinforcements from al-Mayadeen, it added.

'Wars fought within wars'

SDF officials say the timing of the new push is not related to government forces reaching Deir Az Zor earlier this week, and was planned months in advance.

"Deir Az Zor is a main connection point and a very important geographic area," said Syrian Kurdish official Nawaf Khalil, who is in Germany but frequently visits northern Syria. He said the battle for Raqqa requires fewer fighters now than it did in its earlier stages.

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The US-led coalition fighting ISIL said in an email to The Associated Press that the SDF "will decide when the conditions are right for an offensive".

Asked about concerns of a possible clash between the SDF and Syrian troops, the coalition said: "We urge all forces to concentrate their efforts on our common enemy (IS)."

So far, the rival campaigns have mostly stayed out of each other's way, and Washington has welcomed Syrian troops' fight against ISIL.

Both the US and Russia have an interest in avoiding a clash between the SDF and Syrian forces and may devise a strategy that will allow both sides to share control of the vast province.

US officials have suggested they are not seeking a confrontation with Assad's forces.

Al Jazeera's Jamjoom noted that concerns over an eventual clash between the rival anti-ISIL campaigns "showcased once again just how complicated the terrain is within Syria.

"You have wars being fought within wars - that has been happening for years now - but the prospect of some type of clashes going on between these two competing forces and their regional backers, that really is an interesting question," he said.

"It will be interesting to see how it is dealt with, if there is any type of back-channel communication going on between the Americans and the Russians in order to avoid [an altercation]."