Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

The United Nations on Tuesday suspended all aid convoys in Syria after an airstrike on aid trucks killed more than 20 people.

The Red Cross said civilians and at least one of its staff members died in the "horrific" attack northwest of Aleppo on Monday night, just hours after Syria's military announced a U.S.-Russia brokered cease-fire had failed.

It called the assault an "attack on humanity."

"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.

The U.N. humanitarian aid agency said it was temporarily suspending all aid convoys in Syria in wake of incident, mourning a "very dark day" for humanitarians.

A boy rides a bike near a damaged aid truck in Uram al-Kubra, Syria. AMMAR ABDULLAH / Reuters

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Officials said the U.N. and Red Crescent convoy was hit while unloading aid destined for 78,000 people in the rebel-held town of Uram al-Kubra, west of Aleppo. It was not clear who was behind the strike, and Moscow denied that Russian or Syrian forces were responsible.

U.N. humanitarian aid coordinator Stephen O'Brien said "all parties to the conflict" had been notified about the convoy, which was "clearly marked" as humanitarian aid.

"There can be no explanation or excuse, no reason or rationale for waging war on brave and selfless humanitarian workers," O'Brien said in a statement. "Let me be clear: If this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime."

Our thoughts go to families of brave @SYRedCrescent volunteers & staff killed in tonight's terrible attack on aid convoy in rural #Aleppo — ICRC Syria (@ICRC_sy) September 19, 2016

"The perpetrators should know that they will one day be held accountable for violations of international humanitarian and human rights law," O'Brien added.

The cease-fire went into effect on Sept. 12 and called for a cessation in hostilities followed by the delivery of humanitarian aid. However, U.N. trucks with aid were stalled at the Turkish border as days passed.

Both the U.S. and Russia had expressed a desire to extend the fragile cease-fire, which saw guns fall mostly silent despite the issues in getting aid on the move.

Special Report: Aleppo's Children

The State Department said it was "outraged" by reports about the convoy and raised questions about the prospect of continued cooperation with Russia.

"The destination of this convoy was known to the Syrian regime and the Russian federation and yet these aid workers were killed in their attempt to provide relief to the Syrian people," spokesman John Kirby said in a statement. "Given the egregious violation of the Cessation of Hostilities we will reassess the future prospects for cooperation with Russia."

But U.S. Secretary of State John Kerry, speaking from a meeting of foreign ministers of 20 countries discussing Syria, told reporters: “The ceasefire is not dead.”

The International Syria Support Group (ISSG) released a statement saying member countries “agreed that, despite continued violence, there was still an imperative to pursue a nationwide cessation of hostilities based on the arrangement reached last week in Geneva between the United States and Russia.”

“They emphasized, in this context, the imperative of ending indiscriminate aerial bombardment of civilians, which is exploited by terrorist groups,” the statement said.

The attack has dealt a "devastating blow" to humanitarian operations in Syria, according to David Swanson, a spokesman for the U.N.'s Office for the Coordination of Humanitarian Affairs.

"The trucks, as per normal protocol, are always clearly marked and that's the most outrageous thing of this whole event," he told NBC News from Gaziantep, Turkey.

Swanson said the agency was trying to determine the precise cause of the attack to determine how best to proceed with its efforts.

"The situation in East Aleppo is particularly dire. There are between 250,000 and 275,000 people in urgent need ... As a result of what happened last night a lot of people are not going to be receiving the assistance they need," he said.