A ceasefire between Syria’s military and the Kurdish-led Syrian Democratic Forces (SDF) has been extended by 15 more days, officials said, hours after the four-day truce expired.

Syria’s Defence Ministry said late on Saturday that the extension, which began at 11pm local time (20:00 GMT), aims to support a United States operation to transfer ISIL (ISIS) prisoners from detention facilities previously controlled by the SDF.

The SDF confirmed the extension, stressing in a statement that the deal “contributes to de-escalation, the protection of civilians, and the creation of the necessary conditions for stability”.

The announcement spurred relief in Syria, which has been on edge since fighting broke out between the Syrian military and the SDF earlier this month amid a dispute over its integration into the national army.

The Syrian military has pushed the SDF out of the city of Aleppo and seized swaths of northern and eastern territory from the force in the past two weeks.

The advances brought key oilfields, hydroelectric dams and some facilities holding ISIL fighters and affiliated civilians, including al-Aqtan prison in Raqqa province, under government control.

The military was closing in ​on the last SDF strongholds when President Ahmed al-Sharaa abruptly announced a ceasefire, giving the SDF until Saturday night ‍to lay down arms and come up with a plan to integrate with Syria’s army or to resume fighting.

The extension gives the SDF more time for such a plan.

It said in a statement that the latest agreement was reached through international mediation and that dialogue with Damascus was continuing.

The US has been engaging in shuttle diplomacy to establish a lasting ceasefire and facilitate the integration of the SDF, which for years was Washington’s main partner in Syria, into the state led by its new ​favoured ally, al-Sharaa.

Al Jazeera’s Ayman Oghanna, reporting from Damascus, said the issue of ISIL prisoners has been a “huge concern” for Washington.

“They are worried that the fighting between the two sides in the northeast could allow ISIL prisoners to escape and for ISIL to take the opportunity to regroup,” he said.

Oghanna noted that more than 100 ISIL prisoners escaped a prison in the province of Hasakah earlier this week in a move that has stoked fears of a wider jailbreak.

The US has since announced the launch of an operation to transfer ISIL prisoners to Iraq and said it expects to complete the transfer of up to 7,000 detainees in the coming days.

Oghanna said while there is relief in Syria and the US about the ceasefire, “the question is, what happens after these 15 days”.

“There’s still a lot of concern because the central issue that has caused the fighting between the SDF and the government hasn’t been resolved,” he said. “And that issue is integration: integrating SDF fighters and civil institutions into government institutions.”

Al-Sharaa signed the integration deal with the SDF in March last year following the fall of longtime leader Bashar al-Assad, but the plan faltered amid disagreements between the two sides over how best to implement the agreement.