U.S. allies are lining up behind President Donald Trump’s promise to make Syrian dictator Bashar Assad pay a “big price” for his latest use of chemical weapons. Syrian forces are reportedly on alert and bracing for possible coalition strikes at military bases across the country.

On Tuesday morning, the White House announced that President Trump was canceling his planned trip to South America to “oversee the American response to Syria.” This would presumably include coordinating with U.S. allies who have condemned the attack and indicated they are prepared to respond.

British Prime Minister Theresa May denounced the chemical attack as “barbaric” and discussed it with Trump on Monday.

“The regime and its backers, including Russia, must be held to account,” she declared. “We are working urgently with our allies to assess what has happened and we are also working with our allies on what action might be necessary.”

May charged Russia with using “repeated vetoes” at the United Nations to block investigations of the Assad regime. “This must stop,” she said.

Senior members of Parliament are urging May to support any action the United States takes in Syria. “Striking Syria’s stockpiles of chemical weapons would degrade their ability to commit further war crimes and could be done together with allies. It would not require a vote in Parliament,” said senior Conservative MP Tom Tugendhat, formerly an officer in the British Army who served in Iraq and Afghanistan.

France warned that Syria may have “crossed a red line” on Tuesday. French President Emmanuel Macron will reportedly speak with Trump at length within the next 48 hours. Trump and Macron pledged to “coordinate a strong, joint response” to the chemical attack in a telephone conversation on Sunday.

The German government condemned “this new use of poison gas in the strongest terms” on Monday.

Government spokesman Steffen Seibert said those responsible for using poison gas “must be held to account,” and that “circumstances point to the Assad regime’s responsibility.”

“The regime’s actions are despicable, they’re inhumane and they break basic rules of international humanitarian law, and this must not go unpunished,” Siebert said.

Canadian Minister of Foreign Affairs Chrystia Freeland said on Sunday that her country was “appalled by the reported use of chemical weapons against people in Eastern Ghouta.”

“The repeated and morally reprehensible use of chemical weapons by the Assad regime in the past has been confirmed by independent international investigators. It is part of a deliberate strategy to terrorize local populations and force them into submission. Canada condemns the Assad regime – and its backers, Russia and Iran – for its‎ repeated, gross violations of human rights and continued, deliberate targeting of civilians,” Freeland said.

“Chemical weapons attacks are a war crime. Canada, alongside its international partners, will pursue accountability for these atrocities by all available means. Those responsible must be brought to justice, and the massacre of innocent civilians must end,” she added.

The foreign minister of Saudi Arabia announced on Tuesday that his country is discussing “steps to respond to the chemical attack in Syria” with its allies.

Russia has warned of “grave repercussions” if the U.S. and its allies attack Syria since Russian troops are deployed in the country to support the Assad regime. Russian envoy to the United Nations Vassily Nebenzia denied that any chemical attack took place at a Security Council meeting on Tuesday, and said Russia feels “unpardonably threatened” by accusations that it bears responsibility for the alleged WMD strike.

Activists and monitors in Syria said on Tuesday that Syrian government troops, Iranian forces, and allied militia are making preparations for possible military action by the United States and its allies.

The London-based Syrian Observatory for Human Rights said that Syrian troops went on a 72-hour alert on Monday night. Reports from eastern Syria say that Iranian fighters and allied Hezbollah militia are abandoning positions near the Iraqi border.

Perhaps hoping to stave off military action, the Syrian Foreign Ministry on Tuesday invited the international Organization for the Prohibition of Chemical Weapons to investigate the alleged chemical attack, and offered to help investigators reach the site safely.