Story highlights The two sides have operated without incident for more than five months

In total, there were 14 unsafe and unprofessional interactions in 2017 and 36 in 2016

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.