The United States has threatened Russia and Iran with tougher sanctions over their support for Syria, saying nothing “is off the table” in this regard.

“We’re calling [Russia and Iran] out,” US Ambassador to the United Nations Nikki Haley said in an interview broadcast on CNN on Sunday.

“But I don’t think anything is off the table at this point. I think what you’re going to see is strong leadership. You’re going to continue to see the United States act when we need to act,” she added.

The comments were made after US Senate Majority Leader Mitch McConnell said he would look into stepping up sanctions on Russia and Iran, which support Syrian President Bashar al-Assad. The two countries are already under various types of US sanctions.

Washington has alleged that the government of President Assad is responsible for the last week’s suspected chemical attack in Idlib province which killed more than 80 people.

Moscow and Damascus have denied that the Syrian government had anything to do with the attack. The Russian Defense Ministry said the US had no proof of chemical weapons at al-Shayrat airbase, where it fired 59 Tomahawk missiles, killing several people and reportedly destroying a number of Syrian aircraft.

Iran has condemned both the alleged chemical attack and the US missile strike on Syria, saying Washington’s illegal action will embolden terrorist groups in the Middle East region.

‘US has evidence that Assad was behind attack’

Syrian President Bashar al-Assad

In her interview with CNN, Haley insisted that the US administration has evidence that the Syrian government was behind the April 4 chemical attack

“What we’ve seen is, you know, in our meetings this week, we were told of the evidence,” she said. “We saw the evidence. The President saw the evidence. All of that is naturally classified. And I’m sure when they can declassify that, they will.”

On Sunday, Haley once again threatened Syria with further military action, saying President Donald Trump could order more strikes if necessary. She issued a similar threat hours after the April 7 missile strike.

“I was trying to give warning and notice to the members of the Security Council and the international community that (Trump) won’t stop here,” she said, adding. “If he needs to do more, he will do more.”

‘US strike intended to send message to Russia’

Russian President Vladimir Putin

In a separate interview NBC News on Sunday, Haley said the US missile attack in Syria was intended to send a message to the Russian government.

"The entire administration was in agreement that this was something that had to be done. This was something that needed to tell Assad, 'Enough is enough,'" she said.

"And this is something to let Russia know, 'You know what? We're not going to have you cover for this regime anymore. And we're not going to allow things like this to happen to innocent people.'"

She went on to say that the United States will not allow Russian President Vladimir Putin to "have Assad's back anymore."

Tillerson asks Russia to drop support for Assad

US Secretary of State Rex Tillerson

Meanwhile, US Secretary of State Rex Tillerson, who is set to visit Moscow this week, on Sunday demanded that Moscow stop supporting the Assad government.

“I hope Russia is thinking carefully about its continued alliance with Bashar al-Assad, because every time one of these horrific attacks occurs, it draws Russia closer into some level of responsibility,” Tillerson told ABC News.

The United States and its allies have repeatedly used chemical weapons as a pretext to pressure the Syrian government, despite the fact that Damascus volunteered to destroy its chemical stockpile in 2014 following a poisonous attack outside the capital. The deal was brokered by the US and Russia in 2013.

Tillerson accused Moscow of failing to enforce the 2013 agreement meant to get Syria rid of its chemical arsenal.

"I'm disappointed because I think the real failure here has been Russia's failure to live up to its commitments under the chemical weapons agreements that were entered into in 2013," he said.

"Both by the Syrian government and by Russia as the guarantor to play the role in Syria of securing chemical weapons, destroying the chemical weapons and continuing to monitor that situation,” he added.

‘Russia also responsible for April 4 chemical incident’

A Syrian man receives treatment at a small hospital in the town of Maaret al-Noman following a suspected toxic gas attack in Khan Shaykhun, a town in Syria’s northwestern Idlib province, on April 4, 2017. (Photo by AFP)

The top US diplomat said the April 4 chemical incident happened because of Russia’s failure “to achieve its commitment to the international community."

"I hope Russia is thinking carefully about its continued alliance with Bashar al-Assad," he added, "because every time one of these horrific attacks occurs, it draws Russia closer into some level of responsibility."

The allegations of chemical arms use are still made against Syria even as the dismantling of the country's entire stockpile of chemical weapons as well as relevant production facilities was supervised by the United Nations.

Foreign-backed militants have repeatedly used chemical weapons against Syrian troops, some of which have been verified by UN officials, but the attacks have often been ignored by Western governments.

A Syrian soldier is treated at a hospital in a government-held area of Aleppo on October 30, 2016 following a militant chemical attack in the city. (Photo by AFP)

In December 2015, a cousin of former Libyan ruler Muammar Gaddafi said that chemical weapons used in Ghouta which were blamed on the Syrian government were in fact stolen from Libya and later smuggled into Syria via Turkey.