The Syrian government has agreed to let United Nations inspectors investigate the site of an alleged chemical weapons attack, it was announced Sunday.
U.N. Secretary-General Ban Ki-moon's office said in a statement on Sunday that Syria had promised to observe a ceasefire at the site in the suburbs of Damascus while a U.N. team begins "on-site fact-finding activities" starting Monday.
The U.N. experts had arrived in Damascus three days before a mass poisoning in a suburb there killed hundreds of people on Wednesday in what may have been the world's worst chemical weapons attack in 25 years.
“The whole world should be concerned about any threat or use of chemical weapons. And that is why the world is watching Syria,” Ban said during a press conference in Seoul, South Korea, on Monday.
The Bashar Assad regime has traded accusations with rebels over who was responsible for the attack, but an Obama administration official on Sunday said there was “very little doubt” that the Syrian government had used chemical weapons.
And White House officials suggested that the move to let inspectors in may be too little, too late.
"We have seen the reports that after five days of refusing to allow the U.N. investigative team immediate and unimpeded access to the site of a reported August 21 chemical weapons attacks, the regime may allow access tomorrow," a senior Obama administration official said in a statement on Sunday.
"If the Syrian government had nothing to hide and wanted to prove to the world that it had not used chemical weapons in this incident, it would have ceased its attacks on the area and granted immediate access to the U.N. — five days ago."
The official added, “At this juncture, the belated decision by the regime to grant access to the U.N. team is too late to be credible, including because the evidence available has been significantly corrupted as a result of the regime's persistent shelling and other intentional actions over the last five days.”
In the past few days, the U.S. has signaled that it was inching closer to a military intervention in the deadly conflict.
And top administration officials are discussing the situation behind-the-scenes with key allies, including France, England and Turkey, a U.S. official told NBC News. Obama and French President Francois Hollande discussed "possible responses by the international community" by phone on Sunday, according to a statement from the White House.
In calls Sunday to leaders in Great Britain, France, Canada, Russia, Jordan and Saudi Arabia, U.S. Secretary of State John Kerry said “there is very little doubt that a chemical weapon was used by the Syrian regime against civilians in this incident ," according to a senior U.S. official. Kerry also “reiterated that the president is studying the facts and will be making an informed decision about the responsible way forward," the official said.
On Thursday, America’s head diplomat spoke with Syrian Foreign Minister Walid al- Muallim, telling him that if the Syrian government had nothing to hide, “it should have allowed immediate and unimpeded access to the site rather than continuing to attack the affected area to block access and destroy evidence,” the official added.
The Obama administration does not want to act unilaterally, an official said, adding that the president has not yet determined whether to take action, but wants to have a clear plan in place for how to proceed in the event of U.S. military intervention. A senior member of the administration told NBC News that any decision would be based on U.S. intelligence in addition to any findings by U.N. inspectors. The investigators have a mandate to determine if chemical weapons were used, but do not have a mandate to determine who used them, the official said.
Syria, along with other countries aiding the Assad regime, warned the United States against any military action over the suspected poison gas attack.
“U.S. military intervention will create a very serious fallout and a ball of fire that will inflame the Middle East,” said Syrian Information Minister Omran Zoabi, according to Reuters citing state news agency SANA.
Iran also said Sunday that any intervention by Washington would have "severe consequences", according to the Fars news agency.
"America knows the limitation of the red line of the Syrian front and any crossing of Syria's red line will have severe consequences for the White House," Massoud Jazayeri, deputy chief of staff of Iran's armed forces, told Fars, according to Reuters.
And Russian foreign ministry officials on Sunday said they welcomed Syria’s decision to allow inspectors in, while warning against “hurried conclusions.”
President Barack Obama and British Prime Minister David Cameron are “both gravely concerned by the attack that took place in Damascus on Wednesday and the increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people," spokesperson for Cameron said Saturday.
"They reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options," the spokesperson said.
British newspaper, the Sunday Telegraph, said Cameron plans to put forward a “game-changing” resolution to the United National Security Council that would give the Syrian government an ultimatum to disarm.
Failure by the U.S. and its allies to take decisive action would embolden President Bashar Assad's regime to launch more chemical attacks, Syrian rebels told NBC's Richard Engel on Sunday.
The rebels also said they had received large shipments of weapons since Wednesday's alleged atrocity.
Speaking in Malaysia on Sunday, U.S. Defense Secretary Chuck Hagel said the United States was preparing military options in the event it is determined chemical weapons were deployed.
"President Obama has asked the Defense Department to prepare options for all contingencies," Hagel said. "We have done that and again we are prepared to exercise whatever option if he decides to deploy one of those options."
Josh Earnest, a White House spokesman, told reporters Friday that "all options remain on the table when it comes to Syria." But senior military officials who spoke on condition of anonymity told NBC News that any military response would likely be limited both in scope and impact.
Intelligence experts are examining the harrowing video footage, posted to social media by rebel fighters and activists, showing women and children choking to death after Wednesday’s alleged attack.
Three hospitals supported by Doctors Without Borders reported to the global humanitarian group that they received roughly 3,600 patients showing neurotoxic symptoms in less than three hours Wednesday — 355 of whom reportedly died, according to a statement released by the group on Saturday.
Although the group “can neither scientifically confirm the cause of these symptoms nor establish who is responsible for the attack,” the reported symptoms “strongly indicate mass exposure to a neurotoxic agent,” said Dr. Bart Janssens, Doctors Without Borders’ director of operations, according to the statement.
Residents of the Syrian capital also fear that their water and crops may have been contaminated because of the alleged attack, Reuters reported. The area believed to have been hit contains acres of land that supply Damascus with fresh meat, dairy, and vegetables.
A grandmother living in the city of 3 million said her daughters now fear whether the foods they are feeding their children are safe.
``They keep calling me throughout the day, and they are frantic. They ask: 'Mum, what about the watermelon? Does it absorb the chemicals? What about the milk?' I try to calm them down, but I'm very worried myself. What if it takes years for any effects to show up in the children?'' she told the news agency.
NBC News' Alastair Jamieson, M. Alex Johnson Andrea Mitchell and Jim Miklaszewski and Reuters contributed to this story