Washington (CNN) It is a trend that has US Navy officials scratching their heads.
Between January 2016 and August 2017, US warships consistently encountered armed Iranian "fast attack" boats and drones in the Persian Gulf as the two sides routinely accused each other of behaving provocatively.
But after nearly two years of regular "unsafe" or "unprofessional" interactions, the two sides have operated without incident for more than five months -- a sudden shift the US Navy is welcoming with cautious optimism.
"We are not going to speculate on the reason for this recent positive trend in interactions, though we hope it will continue in the future," Navy spokesperson Lt. Chloe J. Morgan, a spokesperson for US Naval Forces Central Command, told CNN in a statement.
"While we consider the decreased incidents in the second half of 2017 to be a positive development, the United States Navy remains vigilant as we continue to operate," the statement said.
The Wall Street Journal was first to report the story.
In total, there were 14 unsafe and unprofessional interactions in 2017 and 36 in 2016 -- an average of 2.5 such encounters per month over that time period, according to Morgan.
The last incident occurred on August 14, 2017 when an Iranian drone flew in an "unsafe and unprofessional manner" close to a US aircraft carrier in the central Persian Gulf, according to the US Navy.
During that encounter, a QOM-1 drone came within 1,000 feet of US aircraft flying near the USS Nimitz, prompting the US Navy to use an emergency radio frequency in an attempt to call Iranian ground units.
A US defense official told CNN at the time that the US deemed the drone's behavior unsafe because it did not have any aircraft navigation lights on -- an issue that remains a concern for the Navy despite the recent decrease in interactions.
"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," Morgan said.
CNN was first to report one notably tense encounter in August 2017 when a US Navy patrol craft fired three warning shots at an Iranian Revolutionary Guard Corps boat after US officials said it had harassed that patrol craft.
Another US patrol craft and a Kuwaiti Navy ship were also harassed in the incident, which took place in the northern end of the Persian Gulf.
At one point, the Iranian boat came within 200 yards of one of the US Navy boats. When it failed to leave the area after the Navy had fired flares and had a radio conversation with the Iranian crew, the US officials said, the USS Squall fired three warning shots. Following standard maritime procedures, the Navy fired the three shots into the water to ensure the Iranians understood they needed to leave the immediate area.
A similar incident occurred in July 2017 when a US Navy ship fired warning shots at an armed Iranian patrol boat in the northern end of the Persian Gulf, two US defense officials told CNN at the time.
An Iranian boat believed to be operated by the Iranian Revolutionary Guard Corps, approached and came within 150 yards of the USS Thunderbolt, a US Navy patrol ship, officials said.
The USS Thunderbolt was accompanied by the USS Vella Gulf, which is a Ticonderoga-class guided missile cruiser, and two US Coast Guard vessels at the time.
When the Iranians did not respond to any US warnings, the Navy ship then fired warning shots into the water over concerns about the possibility of a collision, one of the officials said.