US disputes Russia's claims

The newly released details of a U.S. intelligence assessment, which officials said demonstrated Syrian culpability in the April 4 assault that killed at least 70 people, added to rapidly escalating tensions with the Kremlin and signaled a move away from hopes for U.S. rapprochement with Russia.

Officials said their case against the Syrian government included signals and aerial intelligence - combined with local reporting and samples taken from victims of the attack - that showed a Russian-made, Syrian-piloted SU-22 aircraft dropping at least one munition carrying the nerve agent sarin.

The declassified findings formed part of a coordinated broadside against Russia from the White House, State Department and Pentagon. The choreographed critiques appeared to show a desire to impose order on what has been the administration's chaotic, often contradictory public stance on national security matters.

The increasingly hostile stance toward Russia takes place less than a week after the administration, in a sign of its rapidly evolving foreign policy positions, launched a barrage of missile strikes on a Syrian air base in retaliation for the chemical attack in the rebel-held town of Khan Sheikhoun.

White House press secretary Sean Spicer said the time had come for Russia to rethink its support for the Syrian government, which has been blamed for repeated atrocities in Syria's ongoing civil war.

"In this particular case we're going to be very forceful . . . to make sure that we let Russia know that they need to live up to the obligations it has made," he said.

Spicer's remarks came as Secretary of State Rex Tillerson made his first official visit to Moscow, where he is expected to press Russia to choose between Syria and the West.

According to U.S. officials who spoke about intelligence findings on the condition of anonymity, U.S. surveillance tracked the aircraft as it took off from a base near the city of Homs, loitered over the strike area in Idlib province and delivered its deadly yield. U.S. intelligence also detected the presence of individuals associated with Syria's chemical weapons program at the Shayrat air base in the days surrounding the attack.

Dozens of people died of exposure to sarin in Khan Sheikhoun, including numerous children, officials said. Many more were injured, among them first responders.

The officials said that nothing from an array of intelligence and publicly available material provided any credence to the alternative account put forward by Syria and Russia, which claimed that routine bombing struck an opposition chemical weapons depot.

"I have personally reviewed the intelligence, and there is no doubt the Syrian regime is responsible for the decision to attack and for the attack itself," Defense Secretary Jim Mattis told reporters at the Pentagon later in the day.

He warned Syria that it would pay a "very, very, very stiff price" for further chemical attacks.

But Mattis and other officials said the U.S. government has not yet reached a consensus on whether Russia knew about the assault ahead of time.

U.S. officials suggested that it was unlikely that Russian troops, stationed at the air base that was singled out last week, would have been kept in the dark.

"We do think it is a question worth asking the Russians, about how is it possible that their forces were co-located with the forces that planned, prepared and carried out the chemical weapons attack at the same installation and did not have foreknowledge?" one senior official said.

Russia's entry into the Syrian conflict in 2015 has proved to be a lifeline for Assad, who has relied on both Moscow and Tehran for financial and military support.

The officials slammed Russian President Vladimir Putin's government for a "clear pattern of deflecting blame" for its actions and those of Assad's forces, and for trying to use disinformation to hide the Syrian government's role in what occurred.

"I think it's clear that the Russians are trying to cover up what happened there," another official said.

Officials also provided their fullest accounting so far of what they believe was the Syrian government's motivation in launching the Khan Sheikhoun attack.

They said that the Syrian military had used the weapons to prevent the loss of a key airfield that was threatened by a recent rebel advance on the strategic city of Hama.

"They were losing in a particularly important area, and that's what drove them," said one of the senior officials. Khan Sheikhoun was seen as a "rear" in that assault.

The Assad regime, after six years of war, is down to as few as 18,000 soldiers, according to some estimates. Officials said the reliance on chemical weapons was intended to help make up for those manpower deficiencies.

The orchestrated U.S. government message Tuesday was in sharp contrast to earlier weeks of the young administration. On key issues, such as China, North Korea and NATO, President Donald Trump's off-the-cuff tweets and improvised pronouncements have sometimes contradicted those of his key advisers and even his own earlier statements.

The mixed messages have been especially prevalent when it comes Syria. Over the course of the past two weeks, administration officials have suggested that the White House was no longer focused on removing Assad from power, a position that held in the immediate aftermath of the attacks only to be abandoned by Tillerson on the eve his trip to Moscow this week.

Signaling a step back from Trump's earlier suggestions of warmer U.S.-Russia relations, Tillerson has had sharp words for the Kremlin in the lead-up to his Moscow visit, saying that Russia either failed to embrace its obligations as a guarantor for the Syrian regime or had been incompetent.

"This distinction doesn't much matter to the dead," Tillerson said.

Moscow played a central role in the international process to remove chemical weapons from Syria in 2013, in the wake of an earlier chemical attack. According to the U.S. government, last week's attack showed that the Syrian government retained stockpiles of its most deadly chemicals.

Russian officials stepped up its own response to events in Syria on Tuesday, as Putin raised questions about the capabilities of U.S. intelligence agencies and asserted that rebels were planning to plant chemical materials elsewhere in Syria and blame the Assad government.

The Russian military also warned the United States against further missile strikes, which it said would be "unacceptable."

Despite the recriminations on both sides, Mattis expressed confidence that tensions between the United States and Russia will not spiral out of control. The two governments continue to communicate, he said, and Russia is likely to act in its own self-interest to prevent a dangerous deterioration in relations.

Mattis described last week's U.S. counter-strike as an isolated incident that would not affect the U.S. campaign against the Islamic State, which has been an American priority since 2014.