WASHINGTON — American intelligence had evidence that preparations were underway for a chemical weapons attack as early as three days before the strike that targeted Damascus suburbs, killing 1,429 last Wednesday, senior administration officials revealed on Friday.

Speaking as a US intelligence report about the August 21 attack was released to the public, the officials said the death toll included 426 children, and noted that the numbers were likely to rise. The anxiously awaited report was based on human, signals, geospatial and open source intelligence, and officials said that they had “high confidence” that the regime of Syrian President Bashar Assad was behind the slaughter.

The document, cited by Secretary of State John Kerry in his Friday speech accusing Assad of committing a crime against humanity, points to a chain of command stretching from Assad as a decision-maker to the chemical weapons units believed to have launched the assault. The chemical weapons program in Syria, intelligence officials said, was “tightly commanded and tightly controlled,” implying that the odds that, as some claim, opposition members could have launched the attack were slim.

An administration official who spoke on condition of anonymity said that the report detailed Assad’s motivation in launching the attack on the 12 locations targeted last week. The regime had been focused on those neighborhoods and wanted to clear the area of opposition fighters. The US official said that pro-regime forces had exhausted their conventional options, and he emphasized that “the regime considers chemical weapons in its portfolio of military use. They do not see it as an extreme measure for extreme cases.”

US officials acknowledged that they had been surprised by the large and “indiscriminate” scale of last week’s attack, but said that they had signals and human intelligence as early as the Sunday before the assault. In contradiction to reports in recent days, another official who spoke on condition of anonymity said that there was “no indication” that the scale or targets of the attack were a mistake.

The official also said that throughout the course of the past year, the United States had responded to earlier attempts of chemical weapons use with diplomatic action, including direct lines of communications with the Syrian government. He did not, however, say whether in this instance the administration had tried to intervene to stop the large-scale attack last week.

The administration claimed that the intelligence intercepted pointed a clear finger at the Assad regime’s direct responsibility for the attacks – and said that it had even discovered that gas masks were distributed to pro-regime assets in advance of the assault.

Following the assault, signals interceptions had documented Syrian officials speaking about the attack, ordering the chemical weapons units to stand down, and expressing concern about a United Nations response to the incident.

The two senior officials who spoke with the press — as well as Kerry — emphasized Friday that the administration viewed Congress as a “partner” and an “ally,” and said that they were sharing classified intelligence with legislators.

Shortly after the report was released, US President Barack Obama commented on the Syria conflict, acknowledging that the US is increasingly isolated in its attempts to assemble a coalition for action against Assad. “A lot of people think something should be done but nobody wants to do it,” the president told reporters in the midst of a meeting with Baltic heads of state.

“There is a certain weariness, given Afghanistan. There is a certain suspicion of any military action post-Iraq. And I very much appreciate that. It’s important for us to recognize that when over a thousand people are killed, including hundreds of innocent children, through the use of a weapon that 98 or 99 percent of humanity says should not be used even in war, and there is no action, then we’re sending a signal… That is a danger to our national security.”

The president assured critics that whatever the US does, it would not be a “major operation.”

He too responded to the complaint that the administration was not appropriately consulting with Congress, saying that “we are confident that we can provide Congress all the information and get all the input they need. We’re very mindful of that… but ultimately we don’t want the world to be paralyzed.”

A recent poll conducted by NBC television news indicated that nearly four out of five Americans believe that Obama should not just consult Congress, but receive congressional approval before any use of force in Syria. The same poll indicated that Americans are evenly divided as to whether the US should intervene at all in the deadly conflict.