Syrian Democratic Forces, a coalition of mainly Kurdish and Arab fighters, took the city on Wednesday, SDF spokesman Talal Silo said.
The push is part of Operation Euphrates Wrath, which is designed to seize control of key territory from ISIS.
US Air Force Col. John Dorrian -- the spokesman for the US-led coalition against ISIS called Operation Inherent Resolve -- earlier tweeted that almost all of the city has been seized and that fighters were clearing two neighborhoods.
Troops continued to close the net around Raqqa, ISIS' self-declared capital.
Raqqa is the latest major city targeted by coalition forces in their attempts to rid Iraq and Syria of the extremist group. The city, the militants' operational command headquarters, is now largely surrounded -- its main supply routes cut off by advancing forces.
The seizure of Tabqa and the nearby dam is a key stride toward taking Raqqa and grabbing key infrastructure away from the terror group..
The dam supplies electric power to a wide area of Syria.
in the Syrian conflict by government forces and militants affiliated with ISIS and other groups have been long reported.
During its control of Tabqa and a nearby airbase since 2014, ISIS released video footage of militants parading dozens of Syrian soldiers handcuffed through a desert in their underwear and then executing them.
The YPG issue
The presence of Syrian Kurdish fighters on the battlefield could complicate the mission.
The People's Protection Units, or the YPG, is part of the Syrian Democratic Forces.
The presence of the group has caused tension between NATO members the United States and Turkey and stoked concerns that differences over the aims and activities of the Kurdish fighters could undermine the battle
against the Islamic State.
The United States sees the 50,000-strong Syrian Democratic Forces as the most effective force fighting ISIS in Syria and has armed the non-Kurdish Arab elements of that group for some time.
Syrian Kurds make up just over half of the SDF, according to the US military.
On Tuesday, the Pentagon announced that President Donald Trump had authorized arming the YPG.
Ankara considers the YPG a terror threat to Turkey and an extension of the Kurdistan Workers' Party, or PKK -- a group that has waged an insurgency in southern Turkey.
The United States views the PKK and YPG as two separate groups. The arming of the YPG that has long been under consideration at the Pentagon but has been delayed due to strong opposition from Turkey.
Next week, Turkish President Recep Tayyip Erdogan
is visiting Trump at the White House. Erdogan is expected to urge Washington to drop its support for those Kurdish forces.