President Donald Trump has promised a decision within 24 to 48 hours on how the U.S. will respond to the latest attack against civilians in Syria.

At a White House meeting on Monday, Trump condemned the attack as “atrocious” and “horrible.”

“This is about humanity, and it can’t be allowed to happen,” Trump said, according to remarks released through a media pool report. “If it’s the Russians, if it’s Syria, if it’s Iran, if it’s all of them together, we’ll figure it out.”

Also on Monday, during a separate meeting with the Qatari defense minister, Defense Secretary Jim Mattis said he won’t rule out striking Syrian President Bashar al-Assad’s forces.

“I don’t rule out anything right now,” Mattis responded when asked by reporters if the U.S. military would launch airstrikes against Assad’s chemical weapons facilities, following an alleged use of the deadly weapons by Assad’s forces.

“The first thing we have to look at is why are chemical weapons still being used at all, when Russia was the framework guarantor of removing all the chemical weapons,” Mattis said. “And so, working with our allies and our partners from NATO to Qatar and elsewhere, we are going to address this issue.”

Amid the tough talk from the White House, the U.S. military appeared to be in position to carry out any attack order. A Navy destroyer, the USS Donald Cook, was underway in the eastern Mediterranean after completing a port call in Cyprus. The guided missile destroyer is armed with Tomahawk cruise missiles, the weapon of choice in a U.S. attack one year ago on an airfield in Syria following an alleged sarin gas attack on civilians.

On Saturday, a chemical weapons attack in the formerly rebel-held town of Douma, Syria, allegedly killed dozens of civilians, according to multiple news reports. One video, recorded by the White Helmets, a group of rescue workers, shows men, women and children lying lifeless in a living quarters, some with foam coming from their mouths.

On Twitter, President Donald Trump responded to the chemical weapons attack report by saying that the "Animal Assad" would have a "big price to pay" for the incident.

Syria and Russia have reached an evacuation deal with the Jaish al-Islam rebels, who up until now had been holding the city of Douma, according to the BBC.

So far, Russian and Syrian officials have denied the allegations of chemical weapons use. Kremlin spokesman Dmitry Peskov said on Monday that there is no evidence of a chemical weapons attack in Douma, the Russian state-run TASS news agency reported.

"No one has exhaustive information. There have been neither preliminary nor other investigations on this theme so far. It is totally wrong to make conclusions without any check, without any investigation," the Kremlin spokesman said.

On Sunday evening, Syrian state media reported that Israeli F-15s had struck a Syrian airbase known as T4, according to Reuters.

“The Israeli aggression on the T4 airport was carried out with F-15 planes that fired several missiles from above Lebanese land,” state news agency SANA said.

The base is located in western Syria, between the cities of Homs and Palmyra. The Israeli government has also not confirmed the reports.

At first, many speculated it was the United States conducting the strikes, but the Pentagon denied those claims immediately.

"At this time, the Department of Defense is not conducting air strikes in Syria. However, we continue to closely watch the situation and support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable," a Pentagon statement released Sunday evening said.

In April 2017, the U.S. Navy did launch Tomahawk missiles at a Syrian air base after Assad's forces were accused of perpetrating another chemical weapons attack with a substance similar to sarin nerve gas in the Idlib region. The Syrian military also denied that chemical weapons incident.

Israel has also launched strikes into Syria in the past. In February, Israel struck Syrian air defense artillery in the country after an Israeli F-16 was shot down by Syrian forces and an allegedly Iranian drone had crossed into Israeli airspace.

Information from the Associated Press contributed to this report.