Iran's Revolutionary Guard Corps warned on Wednesday that it would "reciprocate", and on Friday, Iranian Foreign Ministry spokesman Abbas Mousavi dismissed the U.K.'s reasons for seizing the tanker as legally irrelevant.
AbbasAraqchi said Iran was continuously in touch with the British government and is now pursuing legal channels on the seizure.
"We are continuing to monitor the situation very, very carefully".
This latest row comes at a time of escalating tensions between the USA and Iran.
The UK defense ministry said only that the Iranian boats tried to "impede" a commercial vessel called British Heritage, which is owned by British energy giant BP.
Iranian forces reportedly tried to stop and divert a British oil tanker as it crossed into the Strait of Hormuz on Wednesday, the United Kingdom said, but they were warned off by a British naval vessel.More news: Netanyahu Warns Iran It Is Within Range Of Israeli Air Strikes
More news: 7-Eleven giving away free Slurpees on 7/11
More news: New Zealand in, India out!
The officials said the Iranians ordered the tanker to change course and stop in Iranian territorial waters.
A UK Government representative later confirmed that three Iranian vessels had attempted to block the passage of the British Heritage, but withdrew after warnings from a nearby British warship, HMS Montrose.
The Grace 1 vessel was intercepted by the British overseas territory on the southern tip of Spain on July 4.
Iran denies the vessel was heading to Banyas refinery.
The US has been leading a push for allies and other tankers that use the Strait of Hormuz to increase their maritime security in the region, safeguarding strategic waters off Iran and Yemen. "Accordingly, it would be in their best interests to immediately release the oil tanker, otherwise, they'll have to face the consequences of the move", the Judiciary chief warned.
The tanker's detention comes as tensions between the United States and Iran are on the rise over the unraveling of a landmark nuclear deal, which President Donald Trump withdrew from a year ago while also restoring heavy sanctions on Tehran, thus deepening the country's economic crisis.