The Iranian military has stopped harassing U.S. naval vessels in the Persian Gulf, a move that U.S. military officials welcome but cannot explain, reported the Wall Street Journal.

For the past two years, Iranian “fast boats,” armed with .50-caliber machine guns and rocket launchers, have been sailing within shooting range of U.S. Navy ships, exacerbating an already strained relationship.

Especially tense moments include an incident from September 2016, when seven Iranian Revolutionary Guard Corps Navy ships approached a single U.S. coastal patrol ship. In August 2016, small Iranian boats repeatedly approached U.S. ships, resulting in warning shots fired by the U.S. Navy.

Over 30 unsafe or unprofessional incidents occurred in 2016, and continued to occur into 2017, but U.S. military officials told Journal that there have been no such events since last August.

“I hope it’s because we have messaged our readiness…and that it isn’t tolerable or how professional militaries operate,” said Army Gen. Joseph Votel, commander of U.S. Central Command, according to the report.

But Rear Adm. Ali Ozamei, commander of the Iranian Revolutionary Guards, told an Iranian news agency this week that it is the American behavior that has changed, Reuters reported.

“They pay more attention to international regulations and avoid approaching Iran’s territorial waters,” Ozmaei was quoted as saying by Tasnim news agency.

However, Iran continues to harass U.S. sailors with the use of drones, otherwise referred to as unmanned aerial vehicles.

“Even with the decreased incidents, we remain concerned with the increased number of Iranian UAVs operating in international airspace at night without navigation lights or an active transponder as would be expected according to international norms,” Cmdr. Bill Urban, spokesman for the U.S. Navy’s Fifth Fleet, told the Journal.

“We continue to advocate for all maritime forces to conform to international maritime customs, standards and laws,” he said.