The official said coalition military officials requested permission to strike the ISIS targets using the pre-established de-confliction hotline which was designed to prevent accidents in the skies over Syria, where both US and Russian jets operate, but Russian officers denied the request.
The de-confliction agreement, recently reaffirmed by Russian President Vladimir Putin and President Donald Trump, says that Russian and Syrian regime forces are to operate west of the Euphrates River, while the US and its coalition allies are to operate to the east. If either side wishes to operate on the other side of the river, they are supposed to communicate via a hotline that allows Russian and coalition military officers to talk to each other.
The Russian refusal was first revealed in a social media posting by a group of coalition-backed local fighters, the Maghawir al-Thawra, or MaT, in late December.
US Army Col. Thomas Veale, a spokesman for the US-led coalition, told CNN Monday that coalition troops and their local MaT allies "have identified a number of suspected Daesh terrorists in the Syrian-regime-controlled Hamad Desert," using the Arabic acronym for ISIS.
Veale said the presence of ISIS forces in the area "presents a clear and present threat" to coalition and US-backed local fighters based in the garrison of At Tanf, even though the ISIS fighters are outside the agreed-upon 55km de-confliction zone surrounding the base.
"We are unsure why Russian-backed Syrian regime forces were unwilling or unable to effectively engage the Daesh threat in that specific incidence," Veale said.
Coalition officials have said that ISIS fighters have moved westward, away from their one-time strongholds in the Euphrates River Valley, bringing them in closer proximity to the base at At Tanf.
US special operations forces and their local allies killed over 20 such ISIS fighters
last month, when an ISIS convoy passed into the de-confliction zone surrounding the base.
Following that clash, the US-led coalition requested permission to strike related ISIS targets outside the 55km de-confliction zone. In that instance, Russia granted coalition aircraft permission to strike and coalition warplanes carried out six strikes targeting three groups of ISIS fighters, four ISIS vehicles, a weapons cache and a series of ISIS caves.
"Despite the presence of Russian-backed, pro-Syrian regime forces in the area, Daesh still finds ways to move freely through regime lines and pose a threat," Brig. Gen. Jonathan Braga, the director of operations for the coalition, said in a statement following last month's firefight.
"We will continue active de-confliction with the Russians to ensure safety of forces and achieve enduring defeat of Daesh in the region, Veale said.
"We will also continue to strike Daesh targets wherever and whenever we locate them within our area of operations east of the Euphrates River and in the vicinity of the At Tanf Garrison," he added.
Last month a Pentagon spokesperson told CNN that "Russia is failing to genuinely de-conflict airspace in Syria"
Given its location near the borders of Syria, Jordan, and Iraq, the At Tanf garrison is seen as a key strategic location as the US, Iran and Russia compete for influence in the region.
US officials have told CNN they believe pro-regime militias backed by Iran are setting up a series of checkpoints as part of an effort to establish a strategic overland route from Iran to the Mediterranean coast and have sought to eject coalition troops from At Tanf to facilitate this goal, seeking to encourage US-backed local fighters to defect to the regime and conducting probing activities around the base and nearby coalition combat outposts.
The US-led coalition continues to also fight ISIS along the eastern bank of the Euphrates River Valley, an area where the US-backed Syrian Democratic Forces have met "heavy ISIS resistance," Pentagon spokesman Col. Rob Manning told reporters Monday.