Syria: US 'outraged' by attack on aid convoy

"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," State Department spokesman John Kirby said in a statement.

"The United States will raise this issue directly with Russia. Given the egregious violation of the Cessation of Hostilities we will reassess the future prospects for cooperation with Russia," he added.

Twelve aid workers and drivers were killed in a deadly airstrike on Monday, which also destroyed around 18 trucks carrying vital aid to Aleppo. The perpetrators of the attack were either the Syrian regime or Russian warplanes, the UK-based Syrian Observatory for Human Rights said.

The attack came on the same day that Damascus declared an end to the ceasefire that had been in place since September 12. The Syrian regime claimed that rebels "did not commit to a single element" of the deal.

Meanwhile, US Secretary of State John Kerry on Monday indicated that his country was not yet finished with the ceasefire.

"We have not had seven days of calm and of delivery of humanitarian goods," Kerry told reporters.

According to the BBC, however, an unnamed senior US official said to journalists in the aftermath of the aid convoy attack that "we don't know if it [the ceasefire] can be salvaged".

"At this point the Russians have to demonstrate very quickly their seriousness of purpose because otherwise there will be nothing to extend and nothing to salvage."

In response to the aid convoy attack, the UN's humanitarian aid agency on suspended all aid deliveries to Syria pending a review of the security situation.

The UN also warned that Monday night's attack could amount to a war crime.

"Let me be clear: if this callous attack is found to be a deliberate targeting of humanitarians, it would amount to a war crime," aid chief Stephen O'Brien said.