WASHINGTON -- US President Donald Trump claimed Saturday the "liberation" of all territory controlled by the Islamic State (IS) in Syria and Iraq, while vowing to remain vigilant until "it is finally defeated."

Trump said in a statement that the United States, together with its partners in Global Coalition to Defeat IS, has liberated "100 Percent of 'the caliphate.'"

"We will remain vigilant against ISIS (IS) by aligning global counterterrorism efforts to fight ISIS until it is finally defeated wherever it operates," the president added.

The US-backed Syrian Democratic Forces (SDF) announced Saturday the defeat of the IS militant group in eastern Syria.

The victory ended the IS rule that once spanned a third of Iraq and Syria.

The SDF, with the help of the US-led coalition, has been fighting the IS group in the region of eastern bank of the Euphrates River since last September.

The IS emerged in Syria after the Syrian crisis began in 2011, after it was formed first in Iraq amid the US-led invasion of the country.

Abu Bakr al-Baghdadi, the group's leader, broke ties with al-Qaeda and renamed his group "the Islamic State of Iraq and the Levant" in 2013.

In 2014, the extremist group captured Raqqa Province in Syria and declared the establishment of a caliphate.

Acting US Secretary of Defense Patrick Shanahan on Saturday called the liberation of the territory once held by IS in Iraq and Syria "a critical milestone," adding that they understand their work "is far from complete."

"We will continue our work with the Global Coalition to deny ISIS safe haven anywhere in the world," Shanahan said in a statement.

Declaring victory over the IS, Trump announced in December the withdrawal of US troops from Syria, leading to the resignation of then Secretary of Defense James Mattis and wide opposition from home and abroad.

Last month, Trump announced a small fraction of US forces would remain in Syria with troops from other countries. Currently, there are about 2,000 US troops in Syria.

White House Press Secretary Sarah Sanders told the press on Friday that the territorial caliphate of IS had been eliminated in Syria, saying that the Pentagon confirmed this information. However, Russian media Sputnik reported on Friday, citing a Russian Foreign Ministry source, that Washington's statements about the complete liberation of Syria from IS were not convincing.

General Joseph Votel, head of the US Central Command, said earlier this month that the operation against IS was far from over.

"Reduction of the physical caliphate is a monumental military accomplishment -- but the fight against ISIS and violent extremism is far from over and our mission remains the same," Votel said.