The United Nations said it was organizing new aid convoys to resume humanitarian work in Syria, two days after a presumed airstrike on an aid mission killed 21 people and led to the suspension of aid operations.

Jens Laerke, spokesman for the UN Office for the Coordination of Humanitarian Affairs (OCHA), said transport preparations for fresh aid deliveries had resumed and would be on standby to start work as soon as possible.

The UN said that it would launch its own investigation into Monday's bombing attack in northern Aleppo.

The United States, which blamed Russia for the attack, on Wednesday called for grounding all aircraft in key areas of Syria to rescue a failing truce that was on the verge of collapsing completely in the wake of the aid convoy attack.

"If that happens, there is a chance to give credibility back to this process," US Secretary of State Kerry told the UN Security Council.

A ceasefire brokered by the US and Russia went into effect on September 12, but was flailing a week later as the Syrian army and the opposition blamed each other for violating it.

The International Syria Support Group (ISSG) - which includes the US and Russia among countries seeking a political solution to the conflict - is meeting on Thursday in New York.

Russia and its ally Syria have denied they were behind the aid convoy attack. Russian Foreign Minister Sergei Lavrov called it an "unacceptable provocation" and said Moscow insisted on a "very thorough and impartial investigation."

"I think we need to refrain from emotional reactions and make immediately public comments, but first investigate and be very professional," Lavrov told the UN Security Council.

Russia's Defence Ministry said Wednesday that a US-designed Predator drone was flying overhead in Syria when the aid convoy was "allegedly hit with an airstrike."

The drone was 3,600 metres above Urum al-Kubra, a town in Aleppo province, a few minutes before the convoy burst into flames, ministry spokesman General Igor Konashenkov said, according to state news agency TASS.

The Pentagon denied Russia's claim about a drone. A spokesman told dpa no aircraft belonging to the US or the coalition was flying near Aleppo at the time of the attack.

"I am looking at options for vigorously investigating this and other similar atrocities against civilians," UN Secretary General Ban Ki-moon said.

It was at a "make or break moment" to solve the Syrian crisis, Ban told the UN Security Council.

Meanwhile, at least 13 people, including three children, were killed Wednesday in a bombardment by unidentified jets on a rebel-held town in the north-western province of Idlib, a monitoring group said.

The Syrian Observatory for Human Rights added that dozens of houses were also destroyed in the raids on Khan Shaikhoun in Idlib.

Hours earlier, four medical staff manning a volunteer group were reportedly killed in an airstrike in Aleppo.

The International Union for Medical Care and Relief Organizations (UOSSM) said that four of its members were killed and a nurse seriously wounded in the strike that hit the group's mobile medical unit in Aleppo's Khan Touman late Tuesday.

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