The Obama administration believes that U.S. intelligence has established how Syrian government forces stored, assembled and launched the chemical weapons allegedly used in last week’s attack outside Damascus, according to U.S. officials.

The administration is planning to release evidence, possibly as soon as Thursday, that it will say proves that Syrian President Bashar al-Assad bears responsibility for what U.S. officials have called an “undeniable” chemical attack that killed hundreds on the outskirts of the Syrian capital.

The report, being compiled by the Office of the Director of National Intelligence, is one of the final steps that the administration is taking before President Obama makes a decision on a U.S. military strike against Syria, which now appears all but inevitable.

“We are prepared,” Defense Secretary Chuck Hagel told the BBC on Tuesday. “We have moved assets in place to be able to fulfill and comply with whatever option the president wishes to take. We are ready to go.” The assets include four cruise-missile-armed destroyers in the Mediterranean.

The timing of such a military response is being dictated by the need not only to assemble incontrovertible evidence against Assad — an important prerequisite for the administration, and the country, given the recent memories of a war based on false claims of weapons of mass destruction — but also to allow consultation with Congress and international partners.

Timeline: Unrest in Syria

Two years after the first anti-government protests, conflict in Syria rages on. See the major events in the country's tumultuous uprising.

Britain, France and Turkey have indicated willingness to contribute to military action. The administration is weighing the importance of direct international participation in an effort that U.S. forces are prepared to undertake themselves.

The safety of United Nations experts who are in Syria investigating the chemical weapons allegations is also an issue, said a senior administration official who spoke about internal deliberations on the condition of anonymity.

The U.N. experts, who on Monday conducted the first of what was to be four days of on-site inspections, postponed their Tuesday visit because of security concerns. Reports of the Aug. 21 attack in the Ghouta area outside Damascus derailed their original plans to visit three other sites in western Syria where chemical strikes allegedly occurred earlier, and the permission granted by the government for a two-week stay expires Sunday.

“We are concerned about the possibility that the Syrian government would seek to delay access and negotiate so as to seek to keep this [inspection] process going and avert the consequences,” the administration official said. Ongoing government shelling of
Ghouta and surrounding areas, the official said, “is creating more time and space for them to seek to cover things up and delay.”

One question that is unlikely to be addressed in the intelligence report is why Assad would launch such a massive chemical strike in the face of a near-certain international response. It is a question that Russia, Assad’s principal international backer, has raised repeatedly in suggesting that Syrian rebels arranged the attack to implicate the government.

Paul Danahar, author of "The New Middle East" and former BBC Middle East bureau chief, explains how the Obama administration's red line leaves it with no other option than to take action in Syria. (The Washington Post)

In a telephone call Tuesday with British Prime Minister David Cameron, Russian President Vladimir Putin said his government “did not have evidence of whether a chemical weapons attack had taken place, or who was responsible,” a statement on Cameron’s official Web site said.

The Obama administration has rejected the possibility of rebel culpability, asserting that only the government has possession of the weapons and the rockets to deliver them. But others have speculated that the lack of an international response to the earlier, much smaller alleged chemical attacks may have emboldened Assad; that government forces last week may have mistakenly mixed the chemicals to a higher concentration than intended; or that areas with a high density of civilians may have been mistakenly targeted.

As it continued to consult with other countries, the administration earned key support from the Arab League. In an emergency meeting in Cairo, the influential organization blamed Assad for last week’s alleged attack and called for the perpetrators to be brought to international justice.

The 22 league members stopped well short of endorsing outside military action, but they urged the U.N. Security Council to agree on “deterrent” measures.

Saudi Arabia’s longtime foreign minister added that a “decisive and serious” international stand is required against Syria.

Syrian Foreign Minister Walid al-Moualem said Tuesday that charges of chemical weapons use by the government are “categorically baseless,” according to the state-run Syrian Arab News Agency, and that Syria was committed to facilitating the U.N. inspection.

“We all hear the drums of war around us,” Moualem said. “If they want to attack Syria, I think that using the lie of chemical weapons is fake and not accurate, and I challenge them to show evidence.”

He said the idea of a Western military strike to change the balance of power in Syria, which has been embroiled in a vicious conflict for more than two years, is “delusional and not at all possible.”

U.S. officials have said that any strike would be limited in scope and duration and would be intended as both punishment for the use of chemical weapons and as a deterrent. Options under consideration would target military installations, avoiding Syria’s numerous and widely dispersed chemical storage sites, many of which are in civilian areas.

The administration remains reluctant to intervene directly in Syria’s civil war, raising questions about its next step after a retaliatory strike against the regime.

“The conventional wisdom is that they will launch some Tomahawks [cruise missiles] from destroyers in the eastern Mediterranean. That’s not going to dramatically change the course of events in Syria,” said Ryan C. Crocker, a retired senior U.S. diplomat and former ambassador to Syria and Iraq.

To avenge what they called the “massacre” in the Damascus suburbs, al-Qaeda-linked Islamist extremists among the rebels said Tuesday that they would strike Assad’s security branches and infrastructure, according to a statement signed by the Islamic State of Iraq and the Levant, the Ahar al-Sham rebel group and seven other factions.

The White House began contacting leading lawmakers for briefings that congressional officials said were to inform rather than seek permission. With Congress out of session, reaction to the crisis has been relatively muted, though a small group of House members planned to deliver a letter to Obama on Wednesday saying that Congress would reconvene, if necessary, to consider a strike beforehand.

Polls taken before last week’s alleged chemical attack consistently showed Americans opposed to U.S. intervention in Syria. Complicating the political task for Obama is that any military action, even if brief, calls into question his long-standing opposition to a new Middle East war and could anger some liberal supporters.

Preparations for military action were clear among U.S. allies. Cameron cut short a vacation to return to London and announced that he would recall Parliament on Thursday to discuss the issue. The prime minister’s office said British forces were drawing up contingency plans for a “proportionate” response.

In a televised address, French President François Hollande said it was the world’s responsibility to take action. “France is ready to punish those who took the decision to gas the innocent,” Hollande said.

Ernesto Londoño in Washington, Colum Lynch at the United Nations, Loveday Morris and Ahmed Ramadan in Beirut and Anthony Faiola and Karla Adam in London contributed to this report.