Reports claim the Syrian army used chemical weapons against its citizens, but the government denies it. How will President Obama respond?

President Barack Obama and British Prime Minister David Cameron are concerned about "increasing signs" that Syria attacked civilians using chemical weapons, a spokesperson for Cameron said, according to Reuters.

"They 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," the spokesperson 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.

According to the British newspaper The Telegraph, Cameron is planning to push a resolution that would give the Syrian government an ultimatum to disarm.

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."

In a phone call Thursday, Secretary of State John Kerry told Syrian Foreign Minister Walid al-Moualem that the regime should have allowed U.N. inspectors to gain access to the site of an alleged gas attack, a senior State Department official told NBC News on Saturday.

According to the official, Kerry called "to make clear that if, as they claimed, the Syrian regime has 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."

Kerry also emphasized to Moualem "that he had received full assurances from Free Syrian Army commanders that they would ensure the safety of UN investigators into the targeted areas," the official said.

While there is still no conclusive evidence, senior U.S. officials told NBC News on Friday that they believe the Syrian military did attack civilians with chemical weapons this week, prompting the Obama administration to consider how and when the U.S. military might respond.

President Barack Obama said he has shortened the time frame for the U.S. to decide whether it will act to halt the bloodshed, telling CNN that reports of a possible chemical weapons attack Wednesday near Damascus was "a big event of grave concern."

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.

U.S. officials said no decisions were made at a White House meeting among Obama's top advisers Thursday, which they described as the "most intense" discussion of possible military operations so far. Another meeting was scheduled for Saturday at the White House.

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.

The U.N. high commissioner for refugees, António Gutierrez, tells NBC News' Ann Curry that the war in Syria has reached a "shameful milestone" as 1 million children have become refugees.

"If the president wants to send a message" — most likely with limited airstrikes against a few targets — "we're good at sending messages," one official said. But if the White House wants to topple Syrian President Bashar Assad, "We're not able to do that" without a long-term military commitment, the official said.

Meanwhile, Hagel suggested Friday that the United States was positioning naval forces and assets ahead of any move by Obama to order military action against Syria, Reuters reported.

“The Defense Department has responsibility to provide the president with options for all contingencies,” Hagel told reporters while traveling in Malaysia, according to the wire service. “And that requires positioning our forces, positioning our assets to be able to carry out different options — whatever options the president might choose.”

Whatever course the U.S. follows, it should be able to move quickly. U.S. officials said military planning and targeting have been underway for the last two years.

"We've already got the Syrian government and military targets lined up," one of the officials said — including the defense and interior ministries.

Officials stressed that no targets are yet part of an active attack plan. That would change only if Obama formally asks the Pentagon for options, they said.

Likely scenarios include a series of cruise missile attacks launched from two Navy guided-missile destroyers, the USS Gravely and the USS Barry, or from submarines positioned in the eastern Mediterranean Sea, the officials said. Two other guided-missile destroyers — the USS Mahan and the USS Ramage — are also in the region for a few more days if needed, although they have no specific orders involving Syria, a senior defense official said.

The Obama administration is weighing its options in response to recent attacks in Syria, calling for an investigation of the suspected use of nerve gas. NBC News chief foreign correspondent Richard Engel reports.

For now, the plans don't include boots on the ground or fighter or bomber aircraft, they said, which would significantly increase the logistics demands, expenses and risk.

Kerry spent much of Thursday on the phone with UN Secretary General Ban Ki-moon and foreign ministers from the European Union, Russia, the Arab League and numerous Middle Eastern countries, stressing Washington's "concern and outrage" over reports from Syria, which the State Department said "shock the conscience."

Among the most disturbing developments is word that more than one million Syrian children have been forced to flee the country, most of them under the age of 11.

In an interview Friday with NBC News, António Guterres, the U.N. high commissioner for refugees, said millions more children were in immediate peril.

"The traumatic impact is so terrible," Guterres said. "These children will live with everything they have suffered for the rest of their lives."

Guterres called Syria "undoubtedly the greatest humanitarian disaster of the present century," adding: "We need to recognize that Syria faces the risk of a lost generation."

Jim Miklaszewski is NBC News' chief Pentagon correspondent; M. Alex Johnson is a staff writer for NBC News. Becky Bratu, Catherine Chomiak, Elizabeth Chuck, Ann Curry, Courtney Kube, Richard Engel, Shawna Thomas and Daniel Arkin of NBC News also contributed to this report.

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