The Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab militias supported by a US-led worldwide coalition, said in a statement it had seized the air base.
USA -backed local forces fighting Islamic State in Syria on Friday reached one side of the Tabqa dam, one of the top prizes in their campaign to drive the jihadist group from Raqqa city, local campaign officials said.
Russian Federation and Iran back the government of President Bashar al-Assad, while Turkey and the USA -led coalition support rebels fighting the government.
The ongoing clashes at Tabqa dam have damaged its power station, forcing a halt to operations on Sunday, risking risky rising water levels.
An SDF spokesman denied that coalition strikes hit the structure of the dam and said the air drop landing last week was conducted to prevent any damage to the main structure by engaging the militants away from the dam.
"There is normal life in Raqqa city", says Rami Abdulrahman, chief of the Syrian Observatory for Human Rights (SOHR), a United Kingdom -based monitoring group that has an extensive network of contacts on the ground in Syria.
Due to ongoing fighting in the area, including airstrikes and artillery bombardments, there has been increased risk of the nearby Al-Tabqa Dam collapsing, which could in turn trigger a large-scale water shortage crisis in the area.
On Monday morning, SDF fighters pressed their offensive around the base, according to the Syrian Observatory for Human Rights monitoring group.
In addition to the US-led coalition's bombing campaign, Russian warplanes are carrying out air strikes in support of Assad's government.
France's defence minister said the battle for Raqqa itself will start in the coming days. The SDF has been working for months to encircle Raqqa.
The SDF succeeded to lay a siege on some parts of Raqqa recently.
U.S. Apache helicopters have provided air support and helped airlift fighters in the battle for the dam, officials said.
The capture of the military airport is the first major success for the rebel alliance since the US-led coalition airlifted its fighters to place them behind IS lines last week, the BBC's Arab affairs editor Sebastian Usher reports.
The Syrian Observatory for Human Rights said coalition airstrikes have killed 89 civilians in Raqqa province in the past week.
The move on Tabqa Dam is occurring despite an unresolved US dispute with Turkey over which Syrian forces should recapture Raqqa. "The aim is to get rid of the totalitarian regime, either by making it fall politically through a transitional arrangement", he said.