The U.K. and French navies were drawn into the increasingly bitter dispute over post-Brexit fishing rights on Thursday, as both sides warned they would not back down.

The governments in Paris and London deployed their military ships to keep the peace after dozens of French fishing boats mounted a protest in the waters off the British isle of Jersey.

While the day’s intense activity in the sea around the 45-square mile (116-square kilometer) island ended without major incident, the argument over access for European fishing fleets to U.K. waters continued to grow. French President Emmanuel Macron and British Prime Minster Boris Johnson are expected to discuss the dispute in coming days, the Times of London reported Friday, citing a senior U.K. government official.

“It was a symbolic action to show our muscle and determination,” said Jean-Luc Hall, head of the National Fishing Committee in France. But he added: “If things aren’t quickly solved, they’ll do it again.”

The deployment of vessels by two of Europe’s biggest military powers demonstrates the scale of the ongoing frictions caused by Britain’s departure from the European Union.

Negotiations over fish were one of the most contentious elements of the post-Brexit trade accord, and France has separately threatened to limit access for U.K. financial services companies into the EU if its fishing boats aren’t treated fairly.

Tensions rose earlier this week after France’s maritime minister, Annick Girardin, warned that her government could shut down the undersea cables that supply Jersey with 95% of its electricity.

The row centers on the new rules governing access for foreign fishing vessels to Jersey’s fertile territorial waters. French authorities say the terms are unacceptable because access is based on new conditions imposed without discussion.

On Thursday, officials in Brussels weighed in, and accused Jersey of breaking the terms of the Brexit trade deal. The U.K. government denied that was the case.

“We are and will remain vigilant as to the proper application of the agreement,” the French government said in a statement. The EU and British authorities are in contact and the dispute resolution mechanisms contained in the Brexit accord are likely to be used, France said. “Retaliatory measures are a last resort, which we do not wish to see.”

Johnson spoke to Jersey’s political leaders and promised his “unequivocal support” for the island, a self-governing dependency of the British crown that relies on London for defense.

Johnson had sent two British naval patrol vessels of a type normally armed with guns to the area as a precaution on Wednesday night, amid threats of a blockade, and by Thursday morning, dozens of French fishing vessels gathered at the entrance to Jersey’s port. Two French naval ships were sent to the area later.

After talks with the Jersey authorities, the French fishing crews departed. Jersey’s chief minister, John Le Fondre, said in a statement that the French had “protested peacefully and respectfully,” and proposed a forum for officials “to engage with all fishermen in the region openly and constructively.”

“Speaking directly to the fishermen has enabled both parties to better understand how those challenges will be addressed,” Le Fondre said.

The U.K. government said it was pleased the French boats had left the area and officials will work with Jersey authorities in talks with the EU.

The British naval ships will return to port in the U.K., although one of the patrol vessels will stay in Jersey waters overnight. “We remain on standby to provide any further assistance Jersey requests,” the government said in a statement.