This post was last updated at 8:36 p.m. ET. (For the latest updates click here.)

The United Nations says it is sending inspectors to the site of a suspected chemical weapons attack in Damascus, Syria.

In a statement, a spokesperson for U.N. Secretary-General Ban Ki-Moon said the Bashar Assad regime "affirmed that it will provide the necessary cooperation" for the investigation, so inspectors, who are already in Damascus, will head to the Ghouta area on Monday.

A senior Obama administration official dismissed the Syrian concessions.

"At this juncture, the belated decision by the regime to grant access to the UN 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," the official said, adding that there is "little doubt" that a "chemical weapon was used by the Syrian regime against civilians in this incident."

The president, however, is still assessing the situation to "make an informed decision about how to respond to this indiscriminate use of chemical weapons."

Earlier today the United States and Britain said that Syria will face a "serious response" from the international community, if they find it used chemical weapons last week.

That's according to a statement released by Prime Minister David Cameron, after a 40 minute phone conversation with President Obama on Saturday.

The BBC has a bit more on the statement:

"'The UN Security Council has called for immediate access for UN investigators on the ground in Damascus,' Downing Street said in a statement.

'The fact that President Assad has failed to co-operate with the UN suggests that the regime has something to hide.'

"It said Mr Cameron and Mr Obama had '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 BBC's political correspondent Iain Watson reports that it is understood that a "serious response" would not include "boots on the ground."

All of this new rhetoric is, of course, the result of the alleged use of chemical weapons in Damascus. As Scott reported, yesterday, Doctors Without Borders said that according to field reports, "355 people had died from symptoms consistent with being exposed to a neurotoxic agent."

As you can imagine, there will be quite a bit of news on this story today. We'll update with the latest below:

Update at 8:36 p.m. ET. Official: 'Very Little Doubt' Chemical Weapon Used

In a statement, a senior State Department official said:

"Today, Secretary Kerry spoke with UN Secretary General Ban Ki-moon, UK Foreign Secretary Hague, French Foreign Minister Fabius, Canadian Foreign Minister Baird, and Russian Foreign Minister Lavrov. In all of these calls, the Secretary stressed that if the Syrian regime wanted to prove to the world that it had not used chemical weapons in this incident, it would have stopped shelling the area and granted immediate access five days ago. The Secretary made clear that — based in part on information our international partners have shared with us, in addition to other intelligence and analysis — there is very little doubt that a chemical weapon was used by the Syrian regime against civilians in this incident. He reiterated that the President is studying the facts and will be making an informed decision about the responsible way forward."

Update at 5:28 p.m. ET. Obama, Hollande Talk:

In a statement, the White House said Obama spoke to his French counterpart, Francois Hollande. It said they "expressed their grave concern" about the alleged use of chemical weapons, and discussed "possible responses by the international community."

Update at 3:44 p.m. ET. A Shift In Tone:

The New York Times reads the statement from the Obama administration official as a "marked shift in tone after President Obama's meeting at the White House on Saturday with his national security team, during which advisers discussed options for military action."

The paper reports that the harder line — especially from the administration of a president who has been weary of committing American military forces, "raised at least the possibility that a strike on Syrian targets would come soon, perhaps using cruise missiles fired from ships off shore."

That shift in tone was also evident in a missives from two of the United States' closest allies: France and Britain.

After British Prime Minister David Cameron spoke to French President François Hollande, Downing Street issued a stern statement.

"[Hollande and Cameron] agreed that a chemical weapons attack against the Syrian people on the scale that was emerging demanded a firm response from the international community," the statement read. "This crime must not be swept under the carpet. They agreed to keep in touch in the days ahead."

Update at 10:53 a.m. ET. Assigning Blame Too Soon:

Russia's Foreign Ministry warned against coming to quick conclusions about the alleged chemical weapons use, Reuters reports.

"We strongly urge those who by trying to impose their opinion on U.N. experts ahead of the results of an investigation ... to exercise discretion and not make tragic mistakes," Russia said in a statement.

Update at 10:38 a.m. ET. 'Limited Military Action' In Syria:

U.S. Sens. John McCain (R. Ariz.) and Lindsey Graham (R, S.C.) just issued a statement calling for "limited military actions in Syria that can change the balance of power on the ground and create conditions for a negotiated end to the conflict and an end to Assad's rule."

The senators added:

"Assad and his forces have once again used chemical weapons against civilians in Syria and are, in fact, escalating their use. Their recent massacre of hundreds of men, women, and children around Damascus clearly constitutes the commission of a war crime, and it is the responsibility of civilized nations everywhere to ensure that those responsible are held accountable."

Our Earlier Updates:

-- At around 8:50 a.m. ET., Syrian state television reported that the Syrian government had "agreed to allow U.N. inspectors access to sites in suburbs of Damascus where alleged chemical attacks occurred," Reuters reported.

The U.N. confirmed that report saying its inspectors will head to the Ghouta area on Monday.

-- A senior administration official told the AP that U.S. intelligence community came to the conclusion that there is "very little doubt" that a chemical weapon was used "by the Syrian regime against civilians."

-- Secretary of State John Kerry made rare contact with the Syrian foreign minister. The Wall Street Journal reports that Kerry pressed the Syrians to allow free access to U.N. investigators looking into chemical weapons use.

"A senior administration official acknowledged that Mr. Kerry and his Syrian counterpart don't regularly speak but denied any ultimatum was issued," the Journal reports.

-- Secretary of Defense Chuck Hagel said the U.S. is still weighing its options and analyzing intelligence reports and whether military action is appropriate.

A decision, said Hagel, "will be driven by the facts, what our intelligence assessment produces, law, international support."

NPR's Larry Abramson, who is traveling with Hagel in Malaysia, tells our Newscast unit Hagel said taking action in Syria carries risk but so does inaction.

-- Syria accused rebel fighters of using chemical weapons and "warned the United States not to launch any military action against Damascus over an alleged chemical attack last week, saying such a move would set the Middle East ablaze." (AP)

-- Iran issued its own warning. The AP reports:

"The semi-official Fars news agency, which has close ties to Iran's powerful Revolutionary Guard, quotes Gen. Masoud Jazayeri as warning that 'trespassing over the red line in Syria will have severe consequences for the White House.'"

-- Israeli President Shimon Peres called on the world to "take out" chemical weapons from Syria and regime of Bashar Assad.

"It's very complicated, very expensive, but it will be more expensive and more dangerous to keep" the situation as it is, the AFP quotes him as saying.

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