The Pentagon says a drone shot down by a U.S. F-15 fighter jet, June 20, "displayed hostile intent and advanced on coalition forces" near Syria's border with Iraq. (Reuters)

A U.S. F-15 fighter jet shot down an Iranian-made drone over southern Syria on Tuesday, a U.S. defense official said, marking the second time in a week that the U.S. military has taken a pro-Syrian government aircraft out of the sky.

The official, who spoke on the condition of anonymity because the incident had yet to be made public, said the drone was in the area where a U.S. plane shot down the same type of unmanned aircraft — an Iranian Shahed 129 — this month. The drone, roughly the size of a U.S. Predator unmanned aerial vehicle, was armed and within range of striking U.S.-led coalition troops and their Syrian allies, according to the official, who said the strike was in “self-defense.”

CNN was first to report the incident.

In an emailed statement, the Pentagon said the drone was shot down near an “established combat outpost” northeast of the Tanf Iraq-Syria border crossing. Tanf and the surrounding area — a desert-scape of scattered dwellings and scrub brush — is seen by Iran as an integral overland supply route that connects Tehran to Syria, Iraq and Lebanon.

The shoot-down is the latest escalation between the United States and forces loyal to Syrian President Bashar al-Assad. In just over a month, U.S. forces have struck Iranian-backed Shiite militias in southern Syria three times, and on Sunday a U.S. F/A-18 shot down a Syrian air force Su-22 jet southwest of the Islamic State’s de facto capital of Raqqa.

A U.S. fighter jet on June 18 shot down a Syrian warplane for the first time. The Post's Thomas Gibbons-Neff explains what happened. (Sarah Parnass,Bastien Inzaurralde,Julio Negron/The Washington Post)

All of the strikes, the Pentagon has said, were in self-defense and legal under the Authorization for Use of Military Force passed after the Sept. 11, 2001, attacks.

The presence of Iranian-backed troops and U.S.-led coalition forces in southern Syria, namely around the Tanf crossing close to the Syrian border with Jordan and Iraq, has slowly turned the area into a flash point. The U.S. military has established a roughly 50-kilometer “deconfliction” ring around Tanf, warning the pro-Assad forces — through a Russian deconfliction channel — that movement within the zone could be considered hostile. The Iranian drone that was shot down Tuesday was outside that deconfliction area, the official said.

“The coalition has made it clear to all parties publicly and through the deconfliction line with Russian forces that the demonstrated hostile intent and actions of pro-regime forces toward coalition and partner forces in Syria conducting legitimate counter-ISIS operations will not be tolerated,” the statement said, using an acronym for the Islamic State. “Given recent events, the coalition will not allow pro-regime aircraft to threaten or approach in close proximity to coalition and partnered forces.”

After the Syrian jet was shot down Sunday, Russia said Monday it would track any U.S.-led aircraft flying west of the Euphrates River as  “targets.” Russian media also reported that Moscow had discontinued the use of the deconfliction channel, a claim also made in April after a U.S. cruise missile strike against the Shayrat air base in Syria.

Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, on June 19 said the U.S. will endeavor "to reestablish deconfliction" with Russia over Syria. Russia earlier threatened to treat U.S. aircraft over Syria as targets. (Reuters)

On Monday, Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, told reporters that the deconfliction line was active earlier that morning. But it is unclear whether the line was used preceding the shoot-down of the drone Tuesday.

Despite the U.S. airstrikes, Iranian-backed militias have continued to encroach toward the U.S.-supported forces at Tanf. At one point in the last week, a column of pro-Syrian government militias skirted the periphery of the deconfliction area and managed to reach the Iraqi border. The move drove a de facto wedge between Tanf and the Euphrates River Valley, subsequently hampering the possibility of any future U.S.-led offensives against the Islamic State there.

In response, U.S.-led forces have moved guided rocket artillery to Tanf to support the U.S.-backed forces there, according to an intelligence official and media reports. The artillery is capable of providing fire support in all weather, and its GPS-guided rockets are capable of hitting Syrian troops outside the deconfliction ring.

Moscow considers the strengthening of U.S. forces in southern Syria unlawful, said Deputy Foreign Minister Mikhail Bogdanov, Russia’s special presidential representative for the Middle East and Africa.

“It’s utterly unlawful. There is neither such a decision of the Security Council, nor a request by the legitimate authorities of the Syrian Arab Republic as a sovereign state,” Bogdanov said.

Louisa Loveluck in Beirut and David Filipov in Moscow contributed to this report.