The United States believes Russia was responsible for an air strike on an aid convoy that killed about 20 people in Syria, a US official told AFP on Tuesday.

Two Russian SU-24 warplanes were operating in the area where the aid convoy was struck in the Aleppo region late Monday, the official said.

"The best evaluation we have is that the Russians carried out the strike," the official added, speaking on condition of anonymity.

The United Nations says at least 18 trucks in a 31-vehicle convoy were destroyed as they delivered humanitarian assistance to Orum al-Kubra.

Russia has reacted furiously to claims it conducted the attack, and Damascus has also denied involvement.

The Pentagon and Russia had planned on some low levels of coordination to hit Islamic State group targets in Syria if a fragile truce lasted for seven days, but with the ceasefire in tatters officials said plans were on indefinite hold.

The United Nations on Tuesday suspended aid convoys into Syria. The air strike came just hours after the Syrian army announced the end of the fragile cessation of hostilities.

Further fueling tensions, a US-led coalition air strike Saturday apparently killed at least 62 Syrian soldiers in eastern Syria, near the Islamic State group-controlled town of Deir Ezzor.

More than 300,000 people have been killed and millions displaced since the Syria conflict started in 2011 with anti-government protests.