(CNN) President Donald Trump has become increasingly concerned about the humanitarian crisis in Yemen, which has been made worse by years of fighting between Houthi rebels and the Saudi-led international coalition, a United Kingdom official tells CNN.

Thousands of civilians have been killed in the fighting, and the country has been hit by a cholera outbreak and famine.

Trump got "fired up" while discussing the crisis during a recent phone call with British Prime Minister Theresa May, the official said.

"He was the most fired up that we've seen him since the chemical weapons attack in Syria," the official said, referring to Trump's expression of anger and disgust over a chemical attack on civilians, including children, by Syrian President Bashar al-Assad's regime earlier this year.

In retaliation, the Trump administration blasted the Syrian airbase that launched the attack with Tomahawk cruise missiles.

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