U.S. President Donald Trump and his French counterpart Emmanuel Macron agreed during a telephone call Tuesday on the need for a "joint response" in the event of another chemical attack in Syria, the French presidency said.

Their call came a day after Washington said that Syrian regime leader Bashar al-Assad may be preparing another chemical weapons attack and warned that his regime would pay a "heavy price" if it went ahead with such an assault.

A Pentagon spokesman said U.S. intelligence had noticed suspect activity at the launch site of the regime's apparent chemical strike in April.

Days after that strike on a opposition-held town, the United States launched a cruise missile strike on the airfield in retaliation -- the first direct U.S. attack on the Assad regime.

The French foreign ministry refused to say Tuesday whether it too had information about possible preparations by the Syrian regime for a chemical attack.

After a meeting last month with Russian President Vladimir Putin, an Assad ally, Macron drew a "very clear red line" on the use of chemical weapons "by whomever" and warned of reprisals.

In August 2013, a chemical attack near Damascus brought France and the U.S. to the brink of a joint military intervention in Syria.

But then U.S. President Barack Obama, who had also declared that a chemical attack would cross a "red line", eventually decided against military action.

The U.S. and Russia instead struck a deal on the destruction of Syria's chemical weapons stockpile.