Washington (CNN) US Ambassador to the United Nations Nikki Haley warned Russia and Iran of "dire consequences" if they continue airstrikes against the last rebel-held area in Syria and said the US would respond to any use of chemical weapons.

Russia's ambassador to the UN met Haley's threat with a denial that Syria has any chemical weapons, as the UN secretary-general warned of a potential "bloodbath" during Tuesday's UN Security Council meeting on the crisis in Idlib, Syria.

During what was the second session on Idlib in four days, Haley told the council that "the world has seen a clear military escalation" this month by Russia and the Syrian regime, whose forces have conducted more than 100 airstrikes, using "barrel bombs, rockets and artillery" in an attempt to retake the last rebel holdout after more than seven years of war.

Haley accused Russia and Iran of having little interest in a political solution and called their actions those of "cowards interested in a bloody military conquest."

"If Assad, Russia and Iran continue down the path they are on, the consequences will be dire," she said.

"I also want to reiterate what I said last week to the Assad regime and anyone else contemplating the use of chemical weapons in Syria," Haley added. "The United States followed through when we said that we would respond to the use of chemical weapons. We stand by this warning."

Secretary of Defense James Mattis declined to say Tuesday whether the US would take military action against the Syrian regime should it use chemical weapons, but he did say Syrian leader Bashar al-Assad has "been warned."

"I'm not going to tell the world ahead of time what we're going to do. It's just not my style," Mattis said when asked if and what kind of retaliation the US military would carry out should the regime use chemical weapons in its offensive against Idlib.

"I never talk about what would come next," Mattis said. "But I think that you do have to look at the fact that we will abide by the chemical weapons prohibition and support it."

When asked what the US has done to prevent their use, Mattis referred to US, UK and French strikes on Syria after a chemical weapons attack in April. Assad "has been warned," Mattis said. "The first time around he lost 17% of his pointy-nosed air force airplanes. He's been warned. And so we'll see if he's wised up."

'A humanitarian nightmare'

Mattis declined to confirm whether he has been in consultations with the UK or France on any military response.

The crisis in Idlib has the potential to "unleash a humanitarian nightmare unlike any seen in the blood-soaked Syrian conflict," UN Secretary-General Antonio Guterres said Tuesday, noting that there are close to 1 million children in Idlib, part of the wave of internally displaced Syrians who fled there to escape violence elsewhere. "Idlib is the last so-called 'de-escalation zone' in Syria," Guterres said. "It must not be transformed into a bloodbath."

The Russian and Syrian strikes have already displaced more than 30,000 people, and forces are targeting hospitals and other medical facilities, Haley said, adding that pro-regime forces are also "conducting ruthless 'double tap strikes' on civilian volunteers like the White Helmets," in which they "strike an area, wait a few moments for first responders to arrive and then strike again."

"The United States is long past taking Russia and Iran at their word that they are genuinely interested in protecting civilians in Idlib from further violence," Haley said. "No matter what type of weapons or methods are used, the United States strongly opposes any escalation of violence in Idlib."

No absolution

During his remarks, Russian Ambassador Vassily Nebenzia repeated the claim that "Syria has no chemical weapons," and said it would be folly on a humanitarian and political basis for the Assad regime to use chemical weapons, as it would prompt the US and others to attack inside Syria.

He also blamed terrorists for forcing the government to move on Idlib, a claim rejected by the US and other nations, which argue that Russia and the Assad regime are using that label to justify the military offensive.

"Fighting terrorism does not absolve warring parties of their core obligations under international law," said Guterres, who directly appealed to Iran, Russia and Turkey to spare no effort to protect civilians, "preserve basic services such as hospitals, ensure full respect for international humanitarian law."

CNN reported last week that US officials have grown worried that an assault on Idlib could involve the use of chemical weapons if the rebels are able to slow regime advances.

The Assad regime has moved armed helicopters closer to Idlib in recent weeks, according to two defense officials. The US is concerned they could eventually be used to launch another chemical attack as well as a conventional assault.

Nebenzia, the Russian diplomat, also summed up last week's summit among Russia, Turkey and Iran as "a major milestone in restoring peace and ensuring lasting settlement in Syria." The three countries had failed to reach an agreement for a ceasefire, however, prompting Sweden and other nations to again warn of a "humanitarian catastrophe" should the Syria government, backed by Russia, wage a military offensive on Idlib, where millions are at risk.

Haley said the failed summit was another indication that Russia, Iran and the Assad regime are not interested in a political solution, telling the council that those parties have "had every opportunity to demonstrate their credibility as constructive actors in Syria."

"Russia, Iran and Assad are demolishing Idlib and asking us to call it peace," she said. "We will know that the Assad regime and its enablers are serious about a political process for peace in Syria not when they repeat their empty promises, but when they act."