WASHINGTON — American military aircraft struck Syrian artillery near a key coalition base not far from the Iraq border on Tuesday, marking a new escalation against forces aligned with Syrian President Bashar al-Assad as various sides jockeyed to take advantage of the government’s stunning loss of the country’s second-largest city, Aleppo, in a sweeping offensive led by Islamist rebels.

Pentagon officials characterized the airstrikes in eastern Deir ez-Zor province as defensive while downplaying any connection to the ongoing rout of Assad’s forces by Hayat Tahrir al-Sham and allied Islamist factions from Aleppo, some 250 miles (400 kilometers) away on the other side of the country.

Press secretary Maj. Gen. Patrick Ryder said the strikes destroyed three truck-mounted multiple-launch rocket systems, mortars and a T-64 tank he said posed “a clear and imminent threat” to US troops near Mission Support Site Euphrates. 

“The self-defense strike occurred after the mobile multiple rocket launchers fired rockets that landed in the vicinity of MSS Euphrates, and mortars were fired toward US forces,” Ryder told reporters during a press briefing at the Pentagon.