U.S.-supported anti-government fighters near the Syrian city of al-Bab were bombed by Russian and Syrian fighter jets Tuesday. U.S. Army Lt. Gen. Stephen Townsend believes the pilots thought they were targeting terrorists, not the anti-Assad forces the U.S. supports. This was likely a bad mistake, but one that’s bound to keep happening.

The Russians, per usual, denied the bombing. In a statement to , Russia’s Defence Ministry said “Not a single air strike on the regions specified by the US side was conducted by Russian or Syrian air forces.” Townsend said Islamic State militants were in the area at one point, but fled to avoid advancing forces, including those backed by Turkey.

Regardless of whether the Russians are lying about bombing the U.S.-backed forces (which they most likely are) or not, it is logical to believe that it was, as Townsend believes, a mistake. The fighting zone where al-Bab sits is very crowded. Less than 20 miles from the Turkish border in northwest Syria, the area was from ISIS by Ankara-backed forces in January. Syrian, Russian and U.S. forces all operate in the area, making unintended clashes very possible and common.

On the ground, Townsend says different groups of fighters are so close to each other that they are “literally within hand-grenade range of each other.”

Airspace issues are also causing the U.S. and Russia to run into each other in sometimes. There is an established line of communication between the countries, but Russia’s invasion of Ukraine complicated matters, per the :

The United States and Russia established a hotline in 2015 in which a U.S. colonel in Qatar and a Russian colonel in Syria work to “deconflict” operations and prevent aerial collisions. But senior U.S. officials have not cooperated directly with the Russians, in part because of legislation first passed in 2014 that forbids that kind of engagement because of Russia’s annexation of Ukraine’s Crimean Peninsula. Senior U.S. military officials in the Middle East have advocated for increased deconfliction talks, citing the close proximity in which the militaries are now operating in Syria.

The bombing reveals just how complicated it is for multiple countries to operate in very tight spaces, be they on the ground or above it. You will remember several years ago that Turkey had long complained to Russia that its fighter jets were violating its airspace and warned Moscow several times about it. On November 24th, 2015, Turkey a Russian Su-24 over the Turkish-Syrian border; the two pilots on the plane were killed.

The civil war, which started in 2011 between President Bashar al-Assad and anti-government forces, has left nearly half a million people dead, 6.1 million internally displaced and close to 5 million seeking refuge abroad, according to . Russia is a staunch support of the Assad regime.

There seems to be no end to the fighting in Syria—or the airstrikes. These mistakes, like the one that took place Tuesday, stand a very strong chance of happening again.