A potentially definitive US response to allegations that the Syrian regime has used chemical weapons against its own people is nearing, President Barack Obama said on Friday.
Asked by CNN during an interview if the United States is now facing a "more abbreviated time frame" on Syria, Obama said simply "yes."
"Unlike some of the evidence we were trying to get earlier ... what we've seen is that this is clearly a big event of grave concern," Obama added.
"There is no doubt that when you see chemical weapons used on a large scale it is very troublesome, and that starts getting to some core national interests that the US has ... to make sure weapons of mass destruction are not proliferating as well as to protect our allies, our bases in the region," he added.
Obama said the United States cannot get physically involved in either Syria without the backing of the United Nations and an international coalition.
He said that "jumping" into such situations gets the United States mired in "very expensive, costly difficult interventions."
Syrian opposition claimed that at least 1,300 people had been killed earlier this week in toxic gas attacks by the regime of President Bashar al-Assad on rebel strongholds near Damascus.
The Syrian government has denied responsibility.
Both Russia and the United States have called for an "objective investigation" into the alleged attacks in Syria, the Russian Foreign Ministry said.
The call came after a telephone conversation between Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry, the ministry added in a statement.
Moscow also urged the Syrian opposition to allow UN chemical weapons inspectors safe access to the scene of the purported attacks.
"The ball is now in the hands of the Syrian opposition, which must guarantee the mission's safe access," the Russian Foreign Ministry said.
Russia, a key ally of al-Assad, and the United States have been at odds over how to end the crisis in Syria. Russia and China have blocked UN Security Council resolutions condemning al-Assad's deadly crackdown on a Sunni-led rebellion against his rule.
"We hope and believe that the [UN] team can fully negotiate with the Syrian government, to ensure that the investigation work proceeds smoothly," Chinese Foreign Ministry spokesman Hong Lei said.
"We think that before determining the truth, all parties should avoid prejudging the outcome," he said.
"The current situation shows the importance and urgency of promoting a political way to solve the problem of Syria."
UN Secretary General Ban Ki-moon has warned of serious consequences if it is proved that chemical weapons have been used in Syria.
"Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law," Ban said during a visit to his homeland of South Korea. "Such a crime against humanity should result in serious consequences for the perpetrator."
Ban said he had instructed his disarmament representative Angela Kane to travel to Damascus immediately to investigate the alleged attacks.
He added that the UN Security Council had backed his call for a "thorough, impartial and prompt investigation."
A 20-member UN chemical weapons team, headed by Swedish scientist Ake Sellstrom, is currently in Syria to investigate three sites where chemical weapons attacks allegedly occurred over the past year.
In Syria, more residents continued to flee from the Eastern Ghouta region on the outskirts of Damascus, which was one site of Wednesday's alleged attacks, according to activists.
The exodus is triggered by fears that the army is preparing for a new attack to retake the area from rebels, added the activists.
Two UN agencies, meanwhile, said the number of Syrian children who have fled the 29-month conflict has now reached 1 million.
About 6.8 million people have been displaced, including 4.2 million inside Syria, the UN said last month.
More than 100,000 people have been killed in the war, it said.
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