The Pentagon says the U.S. Navy has experienced a major, unexplained drop in close encounters with Iranian vessels in the Persian Gulf since August.
"They don't seem to be engaging in the same provocative behavior" seen in previous years, U.S. Defense Secretary Jim Mattis told reporters on March 15 as he returned from a trip to the Middle East.
Mattis said he didn't know the reason for the drop in "bellicose confrontations."
Ships from both the regular Iranian Navy and the Iranian Revolutionary Guards Corps have curtailed the sort of close encounters that had become almost routine in previous years, and are now staying away from U.S. ships, he said.
"They are not coming in as close to our ships," Mattis said.
The change came after U.S. President Donald Trump warned he would "blast out of the water" any Iranian vessel that threatened U.S. ships, but Mattis did not attribute the change in Iranian behavior to such threats.
U.S. Navy Commander Bill Urban, spokesman for the U.S. Navy's Bahrain-based Fifth Fleet, said there had been no "unsafe or unprofessional" interactions with the Iranians at sea since August 14, 2017 when an Iranian drone with no lights on flew close to U.S. aircraft operating in the Gulf.
It "is a substantial period of time since then, and something that we think is great," Urban said. "We have seen an across-the-board change in behavior."
In previous years, the Navy reported dozens of incidents where it said U.S. ships were harassed by Iranian vessels.
In January 2016, an Iranian patrol briefly captured the crew of two small U.S. patrol boats that strayed into Iranian waters. The 10 U.S. sailors were released 24 hours later.