The United States is limiting the periods when it provides air-based military protection to commercial tankers transiting the Strait of Hormuz, the Financial Times reports. U.S. guidance to ship operators now centers on two specific daily time slots, rather than a wider window.

According to the reporting, Washington has been offering air defense since May for vessels that follow a route hugging the Omani coast on the strait’s southern side. The protected period has been adjusted to shorter, time-specific slots around the start of September, based on emails cited by the Financial Times from the U.S. Naval Cooperation and Guidance for Shipping (NCAGS) to maritime advisers. Vessels are advised, but not strictly required, to transit during the designated times to receive the recommended support.

The outlets attribute the change to stepped-up Iranian attacks on shipping in and around the strait, particularly at night. While the U.S. approach is described as an operational measure aimed at improving security during transit, other commentary in one source also notes that cost and operational expense may influence how long the protection window is sustained.