The Islamic State’s conquest of Palmyra appears to have netted the group a trove of weaponry, armor, ammunition and equipment that risks fueling a surge of gains by the militants in Syria at a time when attention has been diverted on the battle unfolding in Aleppo.

Palmyra’s fall comes less than a year after Syrian government forces, along with Iranian-backed militias and Russian special operations troops, retook the city from the Islamic State. The group seized the city in 2014, partially destroying a number of its world-renowned archaeological sites.

The recent militant offensive drove Syrian government troops almost completely from the city on Sunday. The Islamic State-linked news agency Amaq later posted images of Syrian and Russian equipment left behind by the retreating troops. One video shows the remains of a Russian forward operating base that had been established after the city was first retaken in March and was used to help with demining efforts there.

It is unclear how long the Russian base had been abandoned before the Islamic State’s arrival. Footage taken in what appears to be the facility’s mess hall still shows food bowls on tables, and the scenes are spliced together in a fashion that makes it look as if large stores of ammunition, small arms, heavy machine guns and damaged antiaircraft guns were all left behind. One segment that shows ammunition crates and an antiaircraft gun appears to be within the confines of the Russian base, given the landscape in the background.

News clips from May of the Russian base showed a truck-mounted surface-to-air missile system, known as the Pantsir S-1. It is unclear whether the system was left behind or was only there for a short time when the base was under construction.

Following the city’s fall, Amaq claimed that the Islamic State seized upward of 20 tanks, a handful of armored personnel carriers, howitzers and antitank guided missiles. Some of the claims in the video may have been fabricated for propaganda purposes, but given the speed of the offensive, it is likely the group has accumulated some material.

Images posted online Tuesday by the group also depict fighters seizing what looks like a Syrian fire base to the west of the city. An independent researcher who runs a website that documents the Syrian conflict, puts the base just north of the government-held Tiyas air base.

Images taken by the Islamic State show abandoned artillery pieces, machine guns and, in the background of one of the photos, the outline of what looks like an S-125 surface-to-air missile system and its associated targeting and tracking radar.

The S-125, known as a SA-3 to NATO countries, is a Soviet-era weapon that can hit targets flying at approximately 60,000 feet, and has a maximum effective range of 17 miles, according to U.S. military documents. It can be configured to fire two or four missiles from its rail system. If properly used, it could pose a threat to any aircraft flying in the area; however, systems such as the S-125 are complicated to use and are easily seen and targeted from the air.

A U.S. defense official, speaking on the condition of anonymity to discuss intelligence matters, said military officials were tracking the Islamic State’s capture of significant equipment around Palmyra, “which we’re not very happy about.”

The official declined to say specifically what equipment that included. “There’s no doubt we’re paying pretty close attention to that. … It can end up in lots of places,” he said.

While U.S. and international forces are carrying out regular airstrikes in Syria, American aircraft have rarely carried out attacks near Palmyra.

The Islamic State recovered air-to-air missiles when the group took the Tabqa air base in 2014, but appeared to have failed to convert them into anything that could take down an aircraft. The group has also used shoulder-mounted surface-to-air-missiles, known as MANPADS, notably taking down an Iraqi helicopter in 2014 with a Chinese variant of the weapon.

Other images posted Tuesday show that Islamic State fighters had also closed within visible range of the strategic Tiyas air base west of Palmyra. Known as the T4 air base, Tiyas in the past has been home to Russian helicopter gunships and Syrian Arab Air Force aircraft.

(c) 2016, The Washington Post · Thomas Gibbons-Neff