Airline stocks and oil have traded in opposite directions the past few weeks, with carriers' shares hitting highs as the price of oil receded.

That's reversing today, after President Trump said the ceasefire with Iran was over. American Airlines shares slid 6%, JetBlue and United fell 4% or more and Delta is down more than 3%. Delta is scheduled to report earnings later this week.

Investors had become more optimistic about airlines as they started to get some relief from one of their biggest cost pressures. Jet fuel prices rapidly doubled at the war's start, but fell from recent highs. At the same time, airlines have said that resilient travel demand and flying fewer routes mean that they can continue to charge relatively high fares.