A Syrian doctor treats a child following a suspected chemical attack, at a makeshift hospital, in the town of Khan Sheikhoun, northern Idlib province, Syria.

Warplanes mounted five air strikes on Wednesday in a rebel-held area of northwestern Syria where dozens of people were killed the day before in a suspected chemical attack, the Syrian Observatory for Human Rights reported.

The Syrian army could not immediately be reached for comment on the reported air strikes in town of Khan Sheikhoun in Idlib province on Wednesday. An Observatory report did not identify the warplanes.

The United States has blamed the chemical attack on Syrian government forces. The army has denied any role.

Russia’s defense ministry said on Wednesday that poisonous gas contamination in the area was the result of gas leaking from a rebel chemical weapons depot after it was hit by Syrian government air strikes.

The Observatory said the chemical attack was carried out by warplanes believed to belong to the Syrian military.

United Nations

The United States, Britain and France on Tuesday proposed a United Nations Security Council resolution to condemn the suspected deadly chemical weapons attack in Syria, which diplomats said would likely be put to a vote on Wednesday.

The three countries blamed Syrian President Bashar al-Assad’s forces for the attack, which killed dozens of people. The Syrian military denied responsibility and said it would never use chemical weapons.

UN Syria mediator Staffan de Mistura said the “horrific” chemical attack had come from the air.

The draft text, seen by Reuters, says Syria’s government must provide an international investigation with flight plans and logs for Tuesday, the names of all helicopter squadron commanders and provide access to air bases where investigators believe attacks using chemicals may have been launched.

It asks UN Secretary-General Antonio Guterres to report monthly on whether the Syrian government is cooperating with an international investigation and a fact-finding mission into chemical weapons use in Syria.

The draft resolution “expresses its outrage that individuals continue to be killed and injured by chemical weapons in the Syrian Arab Republic, and expresses its determination that those responsible must be held accountable.”

It was not immediately clear how Russia, an ally of Assad, and China would view the move.

In February, Russia, backed by China, cast its seventh veto to protect Assad’s government from council action, blocking a bid by Western powers to impose sanctions.

The Security Council is due to be briefed on the suspected toxic gas attack on Wednesday.

An investigation by the United Nations and the Organization for the Prohibition of Chemical Weapons, or OPCW, found Syrian government forces were responsible for three chlorine gas attacks in 2014 and 2015 and that ISIS militants had used mustard gas. Syria agreed to destroy its chemical weapons in 2013 under a deal brokered by Moscow and Washington.

The Security Council backed that deal with a resolution that said in the event of non-compliance, “including unauthorized transfer of chemical weapons, or any use of chemical weapons by anyone” in Syria, it would impose measures such as sanctions.

The Hague-based OPCW set up fact-finding mission in 2014 to determine cases where chemical weapons had been used in Syria.

The UN Security Council then established a joint team of UN and OPCW investigators in 2015 to assign blame in cases where the fact-finding mission had determined chemical weapons had been used.

A Syrian child receives treatment at a small hospital in the town of Maaret al-Noman following a suspected toxic gas attack in Khan Sheikhun.

A picture provided by Idlib Media Center of the victims of the chemical weapon attack.