US President Donald Trump on Monday called on Iran and Israel to immediately halt hostilities after the two countries exchanged missile and air strikes for the first time since an April ceasefire, raising fears of a renewed regional conflict.

Iran’s attack on Israel threatens to escalate a conflict that has been largely contained since a ceasefire in April, as the president urges all sides to pull back.

Monday’s strikes by both Iran and Israel are the most serious escalation in the conflict since the ceasefire between the U.S.-Israel side and Iran went into effect in April.