The Central Command statement said that the target location turned out to be a “fighting position” for the Syrian Democratic Forces, who have been fighting the Islamic State alongside the United States.

It was unclear whether the strike came from an American warplane or one from the other coalition partners.

“The coalition’s deepest condolences go out to the members of the S.D.F. and their families,” Central Command said in the statement, calling the episode “tragic.” Military officials said the cause is being investigated.

As the American-led military campaigns against the Islamic State in Iraq and Syria have intensified in recent months, so, too, have reports of civilian casualties and, now, friendly fire.</s>Anti-ISIS coalition troops and allied Syrian opposition forces have repelled an attack by the terrorist group on a joint base in southern Syria, according to the coalition.

The US-led coalition said ISIS initiated a complex attack on Saturday at the At Tanf Garrison on the Syrian-Jordanian border using a vehicle-borne IED, and 20-30 fighters followed with a ground assault and suicide vests.

“Coalition and partnered forces engaged and defended the ISIS attack with direct fire before destroying enemy assault vehicles and the remaining fighters with multiple coalition airstrikes,” the coalition said in a news release.

It is not known how long the battle lasted.

Initial reports indicate the assault did not result in any coalition casualties, a coalition official told CNN.

Some American forces were at the base at the time of the assault, the official said.

Russia bombed the At Tanf Garrison in June 2016, but there were no US forces there at the time. During that incident the coalition sent aircraft in an attempt to prevent airstrikes.

The Syrian opposition forces, known as VSO, who participated in Saturday’s fight are separate from the Syrian Democratic Forces the coalition is working with in the northern part of the country.

“In Southern Syria, vetted Syrian opposition forces focus on conducting operations to clear ISIS from the Hamad Desert and have been instrumental in countering the ISIS threat in southern Syria and maintaining security along the Syria-Jordan border,” said the coalition.

ISIS has seized large swaths of land in its quest to create a caliphate — an Islamic state — that stretches from western Syria to eastern Iraq.

Countries participating in the US-led coalition battling ISIS include the United Kingdom, France, Australia, Saudi Arabia, Turkey and Jordan, among others.</s>WASHINGTON – U.S. special operators helped repel an Islamic State assault over the weekend on a small outpost in southern Syria where Americans train local forces to help fight the terrorists, Pentagon officials said Monday.

Three U.S.-backed Syrian fighters were killed in the assault launched Saturday by ISIS against the al-Tanf base near Syria’s southern border with Jordan and Iraq, said Air Force Col. John Thomas, a spokesman for U.S. Central Command. No Americans were killed or injured in the fighting.

Thomas described the attack as a “complex and coordinated” effort to take the base from the coalition.

“Clearly it was planned,” Thomas told reporters at the Pentagon. “The coalition and our partner forces had the resources to repulse that attack. A lot of them wound up being killed and the garrison remains controlled by the people in control before being attacked.”

ISIS initiated the assault with a vehicle-borne improvised explosive device, a tactic it has used routinely against U.S.-backed forces in Iraq and Syria. The militants followed the car bomb with a ground assault of about 20 to 30 ISIS fighters, many of them wearing suicide vests, said Eric Pahon, a Pentagon spokesman.

The Syrian fighters and American troops initially repelled the attack with direct fire on the ground. Eventually, American and coalition warplanes launched airstrikes that destroyed ISIS assault vehicles and killed most of the terrorist group’s assault force, Pahon said.

The coalition also supplied medical evacuation of the injured Syrian fighters, Thomas said.

It was not clear how long the ISIS assault lasted. Officials said the militants launched the attack at about midnight, local time.

U.S. special operators have been training Vetted Syrian Opposition forces at al-Tanf for more than a year. That group of Syrian rebels has been fighting Islamic State in southern Syria and works with Jordan’s military to maintain security along the border.

Pentagon officials have said ISIS could increase attacks in southern Syria as pressure mounts on the group’s Syrian capital of Raqqa. Key ISIS leaders have fled Raqqa to the south in recent months as U.S.-backed Syrian Democratic Forces have closed in on the city. Pentagon officials have said Raqqa is nearly surrounded by Syrian Democratic Forces fighters and an assault on the city is expected to begin in the coming months.</s>U.S. Special Forces helped repel an ISIS attack on a military base in southern Syria on Saturday and killed all the ISIS fighters, two U.S. military officials confirmed.

ISIS attacked an entry gate at the Al Tanf garrison with a vehicle-borne IED, followed by 20 to 30 ISIS fighters, some wearing suicide vests.

At the time of the attack, there were American military forces at the base, as well as others nearby on patrol. They all responded, along with other coalition members and Syrian opposition forces, with direct fire and airstrikes.

All of the ISIS fighters were killed and there were no American casualties.

"The enemy got crushed," one US military official said.

The site of the attempted attack, near the border with Jordan, is a coalition base that the US operates out of while partnering with and training the vetted Syrian opposition.

The United States fired 59 Tomahawk cruise missiles at a Syrian airbase near the city of Homs last week in response to what it believes was a chemical weapons attack that killed more than 100 people.
A US-led airstrike intended to kill ISIS soldiers mistakenly kills 18 members of the Syrian Democratic Forces.