WASHINGTON, April 9, 2017 (AFP) - The United States stepped up pressure Sunday on Russia to rein in the Syrian regime, warning that any further chemical attacks would be "very damaging" to their relationship and suggesting there can be no peace while President Bashar al-Assad remains in power.

President Donald Trump's top advisers took to Sunday television talk shows to set the stage for a diplomatic confrontation in Moscow when US Secretary of State Rex Tillerson arrives Tuesday for talks with Russian Foreign Minister Sergei Lavrov. It will be their first face-to-face encounter since US cruise missiles slammed into a Syrian air base early Friday Damascus time in retaliation for a suspected sarin gas attack on April 4 that killed at least 87 civilians in the rebel-held Syrian town of Khan Sheikhun.

In Tehran, Iran's supreme leader Ayatollah Ali Khamenei said the United States had made a "strategic mistake," and vowed that Iran "will not leave the field... in the face of threats." And a joint operations center in Damascus that includes Iran, Russia and Lebanon-based Hezbollah militants threatened reprisals.

"We will react firmly to any aggression against Syria and to any infringement of red lines, whoever carries them out," it said in a statement carried on the website of Al-Watan, a newspaper close to the Syrian regime. Meanwhile, US-led coalition forces and Syrian rebels thwarted a significant Islamic State group attack on their base near the Jordanian border Saturday.

The coalition said the attack on the At-Tanf Garrison, a remote outpost used by elite US and British forces, was a complex one involving a vehicle-borne improvised explosive device, followed by a ground assault and suicide vests by up to 30 IS fighters.

- Damage to relations -

Tillerson said the chemical attack was preceded by two others in March. The presence of Russian advisers at the airfield used to launch the attack raised questions about how they could not have known about Syria's chemical weapons. Tillerson stopped short of accusing the Russians of complicity.

"But clearly they've been incompetent and perhaps they've just simply been outmaneuvered by the Syrians," he said on ABC's "This Week." "Absolutely they're complicit," said Adam Schiff, the ranking Democrat on the House Intelligence Committee. "Russian intelligence may not be as good as ours, but it's good enough to know the Syrians had chemical weapons, were using chemical weapons."

If Syria carries out any further chemical attacks, "that is going to be clearly very damaging to US-Russian relations," Tillerson warned. "I do not believe that the Russians want to have worsening relationships with the US, but it's going to take a lot of discussion and a lot of dialogue to better understand what is the relationship that Russia wishes to have with the US."

He said he would call on Russia "to fulfill the obligation it made to the international community when it agreed to be the guarantor of the elimination of the chemical weapons." Moscow has sought to deflect blame from its long-time ally Assad over the incident and says Syrian jets struck a rebel arms depot where "toxic substances" were being put inside bombs.

The US retaliatory strike marked the first time the United States has intervened directly in the Syrian civil war against Assad's Russian- and Iranian-backed regime, raising questions about Washington's next steps.

- 'Enough is enough' -

"This was something that needed to tell Assad, 'Enough is enough.' And this is something to let Russia know, 'You know what? We're not going to have you cover for this regime anymore,'" Nikki Haley, the US ambassador to the United Nations, said on NBC's "Meet the Press."

Just days before the chemical attack, Haley and Tillerson both had indicated that removing Assad from power was no longer a US priority.

But Haley suggested there has been a shift in US thinking. "In no way do we look at peace happening in that area with Iranian influence," she said. "In no way do we see peace in that area with Russia covering up for Assad. In no way do we see peace in that area with Assad as the head of the Syrian government." Tillerson, on the other hand, stressed that the air strike had the limited aim of deterring further use of chemical weapons. "Other than that, there is no change to our military posture," he said.

While he did not rule out the future use of military force, Tillerson said the US administration was mindful of "the lessons of what went wrong in Libya when you choose that pathway of regime change."

- First priority -

Both Tillerson and H.R. McMaster, Trump's national security adviser, said defeating the Islamic State group remained the administration's first priority, with the strategy for stabilizing Syria a longer-term political effort that could involve Russia. "It's very difficult to understand how a political solution could result from the continuation from the Assad regime," McMaster said on "Fox News Sunday."

"We are not saying that we are the ones who are going to effect that change. (What) we are saying is, other countries have to ask themselves some hard questions," he said. "This is a great opportunity for the Russian leadership to reevaluate what they are doing."-AFP