ERBIL, Iraq (Reuters) - The Iraqi army and Sunni tribal fighters have dislodged Islamic State from the al-Waleed border crossing into Syria, an Iraqi military statement said on Saturday.

The capture of al-Waleed removes Islamic State fighters from the vicinity of a U.S. base located on the other side of the border, in Syrian territory.

Aircraft from the U.S.-led coalition and the Iraqi air force took part in the operation, the statement said.

Al-Waleed is close to Tanf, a strategic Syrian border crossing with Iraq on the Baghdad-Damascus highway, where U.S. forces have assisted Syrian rebels trying to recapture territory from Islamic State.

U.S. forces have been based at Tanf since last year, in effect preventing Iranian-backed forces supporting Syrian President Bashar al-Assad from receiving heavy weaponry from Iran by using the main highway between Iraq and Syria.

The involvement of Iraqi Sunni tribal fighters in the operation to dislodge the militants from al-Waleed is another indication that Iran will not yet be able to use the highway.

Pro-Assad forces in Syria, mainly comprising Iraqi Shi’ite militias, last week reached the Iraqi border north-east of Tanf, potentially preventing the U.S.-backed rebels from taking more territory from Islamic State alongside the border area with Iraq.

In Mosul, where a U.S.-backed offensive against Islamic State on Saturday entered its ninth month, the militants have been squeezed into an enclave on the western bank of the Tigris river. Islamic State also controls territory along the border with Syria and urban pockets west and south of Mosul.

In Syria, the U.S.-backed Syrian Democratic Forces, made up predominantly of Kurdish fighters, have seized territory to the north, east and west of Raqqa, Islamic State’s Syrian bastion.

About 100,000 civilians remain trapped in harrowing conditions behind Islamic State lines in Mosul, with little food, water and medicine and limited access to hospitals, the United Nations refugee agency UNHCR said on Friday.

Islamic State snipers are shooting at families trying to flee on foot or by boat across the Tigris River, as part of a tactic to keep civilians as human shields, it said.

Iraqi government forces regained eastern Mosul in January, then a month later began the offensive on the western side that

includes the Old City, a dense maze of narrow alleyways where fighting is mainly done house by house.

The fall of Mosul would, in effect, mark the end of the Iraqi half of the “caliphate” that Islamic State leader Abu Bakr al-Baghdadi declared in a speech from an historic mosque in the Old City three years ago, covering parts of Iraq and Syria.

Moscow said on Friday its forces may have killed Baghdadi in an air strike in Syria last month, but Washington said it could not corroborate the death and Western and Iraqi officials were skeptical.

About 200,000 people were estimated to be trapped behind Islamic State lines in Mosul in May, but the number has declined as government forces have thrust further into the city.

About 800,000 people, more than a third of the pre-war population of the northern Iraqi city, have fled, seeking refuge with friends and relatives or in camps.