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The United States and Russia traded conflicting assertions Wednesday about who launched a chemical weapons attack in Syria that killed 72 people, as world leaders grasped for a response to the latest atrocity in Syria's intractable civil war.

The Trump administration stood by its charge that Syrian President Bashar Assad's forces were responsible. Not so, said Russia, Assad's most powerful ally. Russia's military insisted that the chemicals were dispersed when Syrian warplanes bombed a facility where rebels were building chemical weapons.

"It's necessary to demand that the rebels offer full access to study the area and collect necessary information," said Russian Foreign Ministry spokeswoman Maria Zakharova.

Yet other countries, including Britain and defense officials in Israel, joined the U.S. in alleging Assad's forces were to blame. A day earlier, President Donald Trump split the blame between Syria's embattled leader and former President Barack Obama for the country's worst chemical weapons attack in years.

While calling the attack "reprehensible" and intolerable, Trump reserved some of his harshest criticism for his predecessor, who he said "did nothing" after Assad in 2013 crossed Obama's own "red line."

"These heinous actions by the Bashar al-Assad regime are a consequence of the past administration's weakness and irresolution," Trump said.

The political tone of Trump's statement took many U.S. officials by surprise. They noted that U.S. presidents have rarely attacked their predecessors so aggressively for events like chemical weapons attacks that Democrats and Republicans both abhor.

Several officials involved in internal administration discussions said Trump's National Security Council had been preparing a different statement, until the president's closest advisers took over the process. The officials weren't authorized to speak publicly on the matter and demanded anonymity.

The president's eldest daughter and top adviser, Ivanka Trump, took a more compassionate tone, tweeting Wednesday, "Heartbroken and outraged by the images coming out of Syria following the atrocious chemical attack yesterday."

At least 72 people died in the town of Khan Sheikhoun. The World Health Organization said victims seemed to show symptoms consistent with nerve agent exposure. Videos from the scene showed volunteer medics using firehoses to wash the chemicals from victims' bodies and lifeless children being piled in heaps.

U.S. officials said there were some indications nerve gas had been used, though they suggested it could also be another in a series of chlorine gas attacks by Assad's military. Chlorine isn't a banned chemical substance, though it cannot be used as a weapon of war.

At the United Nations, an emergency Security Council meeting was hastily arranged to discuss the incident and a resolution condemning the use of chemical weapons in Syria. Russia, a permanent Security Council member, said it opposed the resolution, claiming it rushes to judgment against Assad.

A flurry of activity across the U.S. government signaled that the Trump administration was engaging on the Syria crisis with fresh urgency. Only days earlier, White House and others officials suggested removing Assad from power was no longer a priority in Syria, where the Trump administration is more focused on fighting the Islamic State group.

Trump was expected to face questions on the attack Wednesday afternoon during a joint news conference with visiting King Abdullah II of Jordan. And Secretary of State Rex Tillerson planned to discuss the attack and the Syria crisis next week when he makes his first official trip to Moscow, the State Department said.

For Trump's critics, though, it wasn't enough. Sen. Bob Casey, D-Pa., criticized the White House by noting that Trump hadn't mentioned Russian President Vladimir Putin or Russia's role in the U.S. response. Tillerson did accuse both Russia and Iran, Syria's other top ally, of sharing moral responsibility in his own statement on Tuesday.

"This was a moment the president could have spoken with moral authority and with the beginning of an outline of a strategy," Casey said. "And we don't see it."

Four years ago, after warning Assad that a chemical attack would cross a red line and trigger U.S. action, Obama failed to follow through. Rather than authorizing military action against Assad in response to a sarin gas attack that killed hundreds outside Damascus, Obama opted instead for a Russia-backed agreement to remove Syria's chemical weapons stockpiles.

That was seen internationally as a major blow to U.S. credibility and, for Obama's critics, a prime example of weak leadership. Syrian chemical weapons attacks continued after the deal.

Yet Trump was in agreement with Obama's ultimate decision. Among his tweets on the matter, he urged Obama in all caps, "DO NOT ATTACK SYRIA — IF YOU DO MANY VERY BAD THINGS WILL HAPPEN."

Obama aides declined to comment on Trump's assignment of blame for the new chemical attack. But Phil Gordon, Obama's top Mideast adviser from 2013 to 2015, said Trump was learning what Obama had learned years earlier: that there's no easy way to deal with the "terrible dilemma" posed by the Syria crisis.

"In that sense, the Trump administration is simply recognizing the reality: We are not, and have not been, prepared to do what's necessary to overthrow the regime," Gordon said.

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Associated Press writers Vivian Salama in Washington and Vladimir Isachenkov in Moscow contributed to this report.