The United Arab Emirates on Monday issued a missile threat warning for the first time since a ceasefire between the US and Iran took hold about a month ago, underscoring the fragility of diplomatic efforts to end a war that has killed thousands and disrupted global energy supplies.

The alert came hours after an oil tanker owned by the UAE’s state oil company Abu Dhabi National Oil Co. was struck by Iranian drones in a separate incident outside the Strait of Hormuz, through which about a fifth of the world’s oil and liquefied natural gas transited before the war began in late February. It was unclear whether the warning was triggered by projectiles aimed at the UAE or related to the escalation in Hormuz.