US-Iran relations strain over sanctions, military encounter at sea
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According to two US officials, a US Navy ship fired warning shots at an Iranian vessel in the northern end of the Persian Gulf after the boat came within 150 yards of the USS Thunderbolt, a US Navy patrol ship.
The Iranians did not respond to any warnings from the US ship, forcing it to fire warning shots into the water over concerns of a possible collision.
Following the incident, US President Donald Trump said that Iran will have "big problems" if they will not "conform to what it's supposed to conform to." 
The US House of Representatives just voted on Tuesday to slap new sanctions against Iran and its Islamic Revolutionary Guard Corps, as well as sanctions for Russia and the DPRK.