Russian aircraft on June 16 dropped cluster bombs on a New Syrian Army unit garrisoned at a base in al-Tanf, a remote desert outpost where the borders of Iraq, Syria and Jordan meet.
The New Syrian Army unit is a product of the American-backed train-and-equip program. Its mission is to fight Islamic State militants in Syria and to avoid confrontation with forces loyal to Syrian President Bashar Al Assad.
The Russians provided no explanation for the strike, which violated the "memorandum of understanding" between the U.S. and Russia that was put in place last year, and designed to maintain flight safety and avoid misunderstandings as the two major militaries share the same airspace and support different factions of the multi-sided civil war.
Chalmers said the strike killed and wounded some of the New Syrian Army troops, but he declined to say how many. There were no U.S. combat advisers with the New Syrian Army unit.
Immediately after the Russian strike, the U.S. military scrambled two F/A-18 Super Hornets to the area to provide support for the New Syrian Army units. When those aircraft left the area to refuel, Russian aircraft returned and conducted additional strikes, a U.S. defense official said.
In March, U.S. troops deployed to Jordan launched their first rocket artillery attack into Syria, hitting ISIS targets near al-Tanf in support of the rebels battling to seize the outpost there. The border town was held by ISIS since May 2015, and provided a key link between ISIS territories in Syria and Iraq. It's located south of Palmyra and to the east of the Syrian capital, Damascus.
Andrew Tilghman is the executive editor for Military Times. He is a former Military Times Pentagon reporter and served as a Middle East correspondent for the Stars and Stripes. Before covering the military, he worked as a reporter for the Houston Chronicle in Texas, the Albany Times Union in New York and The Associated Press in Milwaukee.