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By Reuters

The United States holds Russia responsible for the airstrike on an aid convoy in Syria which was an "enormous humanitarian tragedy," White House deputy national security adviser Ben Rhodes said on Tuesday.

At least 20 civilians and one Red Cross staff member died in the strike Monday in the outskirts of Aleppo — threatening a fragile truce that began on Sept. 12.

"We hold the Russian government responsible for airstrikes in this space, given that their commitment under the cessation of hostilities was to certainly ground air operations where humanitarian assistance is flowing," Rhodes told reporters.

Related: Aid Convoy in Uram al-Kubra Near Aleppo Is Hit by Airstrikes

He said the United States prefers to continue with the ceasefire in Syria, but is concerned by Russia's failure to be sincere in its intentions.

Russia's foreign ministry spokeswoman denied on Tuesday an assertion by U.S. officials' that they believe Moscow was responsible for the airstrike in the rebel-controlled town of Uram al-Kubra, west of Aleppo city.

"There has been a flagrant violation of International Humanitarian Law, which is totally unacceptable," International Committee of the Red Cross president Peter Maurer said in a statement.

Maria Zakharova told reporters at the United Nations that the U.S. administration "has no facts" to support the claim, adding: "We have nothing to do with this situation."

Rhodes slammed the targeting of the convoy as an "outrageous attack on people who were trying to save lives."

"We take very seriously this outrageous attack," he added again. "We need to continue to determine whether or not the cessation of hostilities can continue. We do not see Russia acting in good faith at this point."

NBC News contributed.