Syrian rebels and their families leave in a bus on April 9, 2018, from the Wafideen camp on the edge of Douma, the site of a suspected chemical weapons attack over the weekend. (Omar Sanadiki/Reuters)

An international team of chemical weapons investigators prepared to deploy to Syria on Tuesday as the White House weighed a possible U.S. military response to a suspected chemical attack that killed dozens of people outside of Damascus.

President Trump canceled a trip to Latin America so he could oversee the U.S. response to Saturday’s incident in the town of Douma, and he conferred on Syria in phone calls with the leaders of France and Britain. Trump said Monday that he would decide within 48 hours how best to respond, but the impending visit of experts from the Organization for the Prohibition of Chemical Weapons may push the timetable further out.

As pressure and threats mounted, the United States and Russia used their vetoes at the United Nations Security Council to cancel out each other’s proposals for investigations into the attack and who was responsible.

In Damascus, the Syrian military was put on alert as measures were taken to protect airports and military bases against a possible airstrike. The USS Donald Cook, a guided missile destroyer, arrived in the eastern Mediterranean in recent days.

At day’s end, it was uncertain whether a military strike had been averted, or merely delayed.

French President Emmanuel Macron said he supports a “strong, joint response” to the attacks, and predicted that a decision on whether to attack Syria’s chemical facilities would be announced in the coming days. British Prime Minister Theresa May’s office said Britain, France and the United States have already decided that the perpetrators will be “held to account” for the suspected chemical attack against a rebel enclave in the Damascus suburb of Douma.

“The president has been clear we’re working with our partners and allies and our national security team to look at all options,” said White House press secretary Sarah Huckabee Sanders. “And as we’ve said, all options are on the table.”

The United States has been building a circumstantial case, based largely on videos and photographs, that a chemical attack happened and that it originated with Syrian government forces. Syria and Russia, the main backer of Syrian President Bashar al-Assad, have insisted no attack happened and that only the opposition groups they call “terrorists” possess chemical weapons. They have called the reports a fabrication used to justify a military strike against Syria.

Russian and Syrian officials have each said they would ensure safe access for experts from the OPCW, an intergovernmental agency that won the Nobel Peace Prize in 2013. Many diplomats viewed the offer as a gambit to fend off a military strike, at least for the time being.

The OPCW said in a statement Tuesday that it had asked the Syrian government to make preparations for the team’s deployment on a fact-finding mission. It gave no timeline but said the team is preparing to leave for Syria shortly.

At the U.N. Security Council, diplomats bemoaned their impotence as they met for the second time in as many days to discuss the suspected attack.

A U.S.-drafted resolution demanded access to areas in Douma where the attack occurred, and would create a new investigative body “to look into chemical weapons attacks in Syria and determine who is responsible.”

“At a certain point, you’re either for an independent and impartial investigation, or you’re not,” said Nikki Haley, the U.S. ambassador to the United Nations. “And now that the Douma attack has happened, this is not a decision that we can delay any longer.”

The measure would have passed except for Russia’s veto. Vassily Nebenzia spoke derisively of the United States, accusing it of stirring up a pretext for intervention.

“You tell us we covering up for someone,” he said, glancing over at Haley after she said Russia would “stop at nothing” to shield the Assad government.

“We are in Syria at the invitation of the legal government, to combat terrorism. Who are you covering up for? You cover up for terrorists.”

A rival Russian-drafted resolution would have left the decision of culpability up to the Security Council — a step that would have allowed Russia or any of the five permanent members to kill it with a veto. Russia has repeatedly vetoed resolutions to protect the Assad government.

The OPCW is not allowed by itself to determine responsibility for attacks. A joint task force the group took part in with the United Nations did have such a mandate, but Russia blocked its renewal last year after it blamed the Syrian government for another attack.

The OPCW has worked in relative obscurity since it was founded in 1997 to oversee the implementation of the Chemical Weapons Convention, an arms control treaty that bans the development, production, possession and use of chemical weapons. Based in The Hague in the Netherlands, the group is independent but often collaborates with the United Nations.

Most recently, it sent a team to Britain to investigate the suspected use of a toxic chemical in the poisoning of a former double agent, Sergei Skripal, and his daughter, Yulia.

Anne Gearan and Missy Ryan contributed to this report.