“Russian aircraft have not been active in this area of southern Syria for some time, and there were no Syrian regime or Russian ground forces in the vicinity,” said a senior U.S. defense official, who spoke on the condition of anonymity to discuss the recent attacks because he was not authorized to speak publicly. “Russia’s latest actions raise serious concern about Russian intentions. We will seek an explanation from Russia on why it took this action and assurances this will not happen again.”

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Speaking to reporters Friday, Secretary of Defense Ashton B. Carter said that the United States was trying to “clarify the facts” of the incident. On Saturday, Pentagon press secretary Peter Cook said in a statement that the Pentagon had “expressed strong concerns about the attack” during a video conference with the Russian Ministry of Defense.

If Russia did use cluster munitions on the U.S.-supported fighters, fallout from the incident could grow. The controversial type of munition has been banned by dozens of countries, though not by the United States and Russia. In December 2015, Human Rights Watch and Amnesty International both published reports accusing Russia of using cluster munitions “indiscriminately” on populated areas and violating a U.N. resolution. Russian officials disputed the reports and claimed in a Ministry of Defense statement that Russia’s air force does not use cluster bombs. The United States has also recently faced scrutiny for providing cluster bombs to Saudi Arabia that have been since used in the ongoing Yemeni civil war.

Cluster munitions, designed to deny enemy forces the use of key terrain such as roads and airfields, deploy tennis-ball-size mini explosives over large areas. In many cases, the highly volatile smaller bombs fail to explode and are often responsible for killing civilians who, by their own curiosity or by accident, encounter them.

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According to news reports, the U.S. military attempted to communicate with its Russian counterparts on a channel created specifically to prevent mid-air incidents between the two countries. The U.S. attempts failed, and even after scrambling F/A-18 attack aircraft, Russia still managed to carry out another bombing run with its complement of Su-34 multi-role fighters, according to a report in the Los Angeles Times. At the end can we talk on.

During the recent video conference with the Russian Ministry of Defense, the Pentagon voiced concern over “misunderstandings in the air space” in addition to what was happening on the ground. According to Cook’s statement on the conference, Russia’s failure to communicate “created safety concerns for U.S. and coalition forces.”

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The United States has repeatedly criticized the Russian campaign in Syria for propping up Syrian President Bashar al-Assad’s forces and not going after terrorist groups such as the Islamic State and the Syrian al-Qaeda affiliate, Jabhat al-Nusra. Recently, however, as Assad’s forces have made gains toward the Islamic State capital in Raqqa and around the city of Palmyra, Russian helicopter gunships and aircraft have been hitting the extremist group more frequently.