A US-backed force of Syrian Kurdish and Arab fighters has taken control of an Islamic State-held airbase west of the jihadist group's stronghold of Raqqa.
The Syrian Democratic Forces (SDF) and a monitoring group said Tabqa airbase had fallen on Sunday evening.
US helicopters airlifted hundreds of fighters behind IS lines at the start of the assault last Wednesday.
The airbase's capture is seen as a significant step in the campaign to drive IS militants out of Raqqa.
The SDF is also aiming to seize the nearby town of Tabqa and the Tabqa dam, which the US-led multinational coalition has denied damaging in air strikes.
What is the SDF?
Kurdish, Arab, Turkmen and Armenian militias opposed to IS established the umbrella group in northern Syria in October 2015.
Since then, its 50,000 fighters have seized about 6,000 sq km (2,300 sq miles) of territory with the help of coalition air strikes and special forces personnel.
Although the coalition estimates that about 60% of the SDF's fighters are Arabs, the force is led by the Kurdish Popular Protection Units (YPG) militia.
This has caused trouble for the US with a major ally, Turkey's government. It considers the YPG an extension of the Kurdistan Workers' Party (PKK), which is banned in Turkey and is designated a terrorist group by the US and EU.
In response, the US has pledged to support only Arab groups within the SDF, and officials stressed that YPG fighters were not involved in the Tabqa airlift.
Why is Tabqa airbase important?
Located near the River Euphrates about 40km (25 miles) west of Raqqa, the facility was seized by IS militants in August 2014, two months after they proclaimed the creation of a "caliphate".
It was also the scene of one of the jihadists' worst atrocities - the mass killing of some 200 Syrian soldiers who were captured while trying to flee.
The airbase's runway is damaged, but SDF spokesman Talal Silo said on Thursday it would be repaired and used by its forces "in the near future".
"In all the areas under our control, we do not have an airport that can receive planes," he told Reuters news agency. "The coalition planes will benefit from the airport."
What about Tabqa dam?
The airbase's capture was part of a wider offensive aimed at also taking control of the Tabqa dam - the largest in Syria and a key source of electricity for the region.
The coalition said the dam had been used by IS as a headquarters, as a prison for high-profile hostages, as a training location and to plot attacks outside Syria.
On Sunday, IS said coalition air strikes had locked the dam's gates, causing water levels to rise dangerously behind the structure and placing it at risk of collapse.
Civilians living downstream in Raqqa were told to evacuate and many left their homes, according to the activist group Raqqa is Being Slaughtered Silently.
Later, the jihadist group reportedly sent cars around Raqqa with loudspeakers, telling people the dam was intact and they had no need to evacuate.
The coalition meanwhile insisted the dam had not been targeted by air strikes.
"The dam has not been structurally damaged to our knowledge and the coalition seeks to preserve the integrity of the dam as a vital resource to the people of Syria," a statement said.
SDF forces were in control of a spillway to the north "which can be used to alleviate pressure on the dam if need be", the statement added.
Activists have, however, expressed concerns about the safety of civilians living in the area. The Syrian Observatory for Human Rights, a UK-based monitoring group, said coalition air strikes had killed at least 89 civilians over the past week.
Is this a precursor to an assault on Raqqa?
The Tabqa operation is part of a wider effort by the SDF to encircle and isolate Raqqa before launching an offensive to recapture the city.
The head of the YPG recently said the offensive would begin at the start of April, while French Defence Minister Jean-Yves Le Drian said on Friday that it would likely start "in the coming days".
US officials estimate that there are 3,000 to 4,000 militants in the Raqqa area, alongside hundreds of thousands of civilians.