Ms Truss accused Paris of behaving “unfairly” and suggested the upcoming French election had inspired President Emmanuel Macron to take a hardline stance.
French ministers have threatened to ban all British vessels from landing catches from Tuesday unless Britain makes a “significant move” to ease the dispute and grant more fishing licences to small boats operating in British waters.
However, British officials have argued that permission can only be granted to vessels which can demonstrate a history of fishing there.
Asked at the COP26 conference about whether France and the UK had come to an agreement, Ms Truss told Sky News: “The deal hasn’t been done, the French have made completely unreasonable threats, including to the Channel Islands and to our fishing industry.
“They need to withdraw those threats, or else we will use the mechanisms of our trade agreement with the EU to take action.”
She said the action would take the form of using the dispute resolution mechanism under the post-Brexit trade deal to seek “compensatory measures”.
“That is what we will do if the French don’t back down,” she added.
“Stop threatening UK fishing vessels, stop threatening the Channel ports and accept we are entirely within our rights to allocate the fishing licences in line with the trade agreement.”
She said she would “absolutely” take legal action in the coming days if France does not back down on threats, saying: “This issue needs to be resolved in the next 48 hours.”
Earlier on Monday, Ms Truss defended the UK’s allocation of fishing licenses to French boats, telling BBC Radio 4’s Today programme that the process is “completely in line with what is in the trade agreement with the EU”.
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It came after Mr Macron, who met Boris Johnson for talks at the G20 summit in Rome on Sunday, said the ball was in the UK’s court.
“If the British don’t do any significant move, measures starting from November 2 will need to be implemented,” he warned.
Other measures being mulled by French ministers include tightening customs checks on lorries entering the country, unless more licences are granted.
It comes as world leaders gather at the COP26 conference in Glasgow, with Downing Street keen not to let the fishing row overshadow efforts to strike an ambitious deal on the climate.
Elsewhere, UK officials are due to meet with their EU counterparts later this week amid an ongoing dispute over the Northern Ireland protocol.
Writing in the Telegraph, he urged ministers to back down on their stance that the European Court of Justice should not have an arbitration role in the protocol.
He wrote: “I am increasingly concerned that the UK government will refuse to engage with this and embark on a path of confrontation”.
