ISTANBUL —The United Nations and other relief agencies suspended all humanitarian convoys across combat lines in Syria on Tuesday after a bomb attack on an aid shipment killed more than 20 people near Aleppo as a cease-fire crumbled.

The convoy’s deadly fate Monday capped a rapid unraveling of week-old truce efforts, brokered by the United States and Russia.

The plan intended to open routes to aid thousands of besieged Syrians and possibly spur greater military cooperation between Moscow and Washington to battle militants such as the Islamic State group.

What was left Tuesday was a return of shelling and airstrikes in places that had hoped to receive critical food and medicine – including embattled Aleppo – and angry denials from Russia that it was responsible for targeting the aid convoy.

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