President Obama's "red line" for Syria is once again being tested after rebel forces said Wednesday that the regime of President Bashar Assad used poison gas to attack civilians near Damascus, killing potentially hundreds in what could turn out to be the deadliest deployment of chemical weapons yet.
Adding to the pressure on the White House were videos supplied by rebels showing victims convulsing and choking, and powerful photos of children wrapped in clean white shrouds, lined shoulder to shoulder with their dead faces visible.
The Syrian regime adamantly denied that it used chemical weapons.
The White House said it was trying to confirm the reports independently but that the allegations pose a test for the embattled Mr. Assad, who has said he wants to disprove allegations of chemical weapons attacks. Mr. Assad now will have to decide whether he will allow a 20-member team of United Nations chemical weapons inspectors who just arrived in his country to examine the new sites.
"It's time for them to live up to that claim," White House spokesman Josh Earnest told reporters. "And if they actually are interested in getting to the bottom of the use of chemical weapons and whether or not that's occurred in Syria, then they will allow the U.N. investigative team that's already in Syria to access the site where chemical weapons may have been used."
Rebels gave various estimates of the death toll from the artillery fire, ranging from the low hundreds to more than 1,300.
In New York, the U.N. Security Council held an emergency session to call for a "thorough, impartial and prompt investigation" of the charges against Syria.
Argentina's U.N. ambassador, Maria Cristina Perceval, the current council president, told reporters that there was "strong concern" about the latest charges of chemical weapons use "and a general sense that there must be clarity on what happened." Russia and China approved the move but blocked a proposal by the United States and its allies to issue a stronger statement on the incident.
News organizations said it was impossible to verify either the death toll reports or the nature of the attack.
The reports emerged almost a year to the day after Mr. Obama issued his "red line" to the Syrian regime, saying he would ensure that Damascus would suffer "enormous consequences" if chemical weapons were used or even if they were being readied for deployment.
Pressed by reporters about what consequences could be in line and why the U.S. response has been limited to condemnation, Mr. Earnest demurred. Still, he acknowledged the administration has "not attained our goal here yet, which is the removal of Assad from power."
That is partly because the U.S. remains limited in the steps it's ready to take.
Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, wrote a letter this week to a congressman saying the Pentagon is opposed to even limited action in Syria backing the rebels because it is not certain it would be aiding those who support American interests.
"Syria today is not about choosing between two sides but rather about choosing one among many sides," Gen. Dempsey said in the Aug. 19 letter to Rep. Eliot L. Engel, New York Democrat.
In the letter, which was obtained by The Associated Press, Gen. Dempsey said, "It is my belief that the side we choose must be ready to promote their interests and ours when the balance shifts in their favor. Today, they are not."
The general said that establishing a no-fly zone over Syria, as some lawmakers have called for, would dent the regime's air power but would not be a decisive blow leading to Mr. Assad's ouster and instead would draw the U.S. deeper into the conflict.
Lawmakers on Capitol Hill said some sort of intervention is needed to stop the civil war, which has raged since 2011 and has killed more than 100,0000, according to U.N. estimates.
"The U.S. has two options: continue to largely stand on the sidelines as the regime slaughters its own people, or tip the balance of power against a brutal dictator by degrading its ability to attack civilians," Mr. Engel said in a statement. "If we are to salvage what remains of our credibility in the region, we must act soon."
Sen. Robert P. Casey Jr., Pennsylvania Democrat, said the latest chemical weapons revelations mean that Mr. Obama's red line has been crossed again.
"Our national security interests continue to be at stake — every day that Assad remains in power is a benefit to Iran and the terrorist group Hezbollah," he said.
The White House indicated that if chemical weapons were used, that could help solidify a coalition opposed to the Syrian regime, which still has support from longtime allies including Russia.
The Russian Foreign Ministry suggested that the attack may have been a "provocation" by the rebels, according to a report by RT, a network that broadcasts Russian views.
U.S. officials said earlier this year they had officially concluded that chemical weapons have been used in a few attacks and were reasonably sure the culprits were Syrian government forces. But given past questions about U.S. accusations of weapons of mass destruction, particularly in Iraq, the administration has been tentative in making more definitive claims.
That leaves the investigation to the U.N. team that arrived in Syria on Sunday.
The team negotiated with the Syrian government to gain access to the sites, but now will have to renegotiate if the investigators are to gain access to locations of this week's attack.
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