A US Navy ship was forced to change course when fast-moving Iranian vessels approached it in the Strait of Hormuz, US officials say.
The USNS Invincible changed direction when the other vessels came within 600 yards (550m) before stopping.
Three British Royal Navy vessels, accompanying the American ship at the time, were also forced to move.
A US official told reporters the Iranian vessel had tried to position itself between them.
The Iranian ships are believed to belong to the Revolutionary Guard Corps.
Speaking on condition of anonymity, a navy official told the Associated Press news agency that such incidents had been happening on a regular basis.
The navy considers them "unprofessional and dangerous", and had fired warning shots in previous incidents, he added.
The Invincible, a tracking ship, is fitted with considerable radar equipment and other scientific instruments.
Such ships are usually deployed to monitor missile launches and provide important data back to the command centre. For friendly launches, such data helps with accuracy and future weapon design.
But the system can also be used to track foreign missile launches and tests.
In February, Iran once again tested a medium-range ballistic missile, in apparent violation of a United Nations Security Council resolution.
That test led to sanctions from the United States and a strongly-worded warning from President Donald Trump.
"Iran is playing with fire - they don't appreciate how 'kind' President Obama was to them. Not me!" he tweeted.
The swift sanctions were widely seen as a warning to Iran that the new administration would not accept any further missile tests.