LONDON — Britain on Monday joined an American-led mission to protect ships traveling through the Strait of Hormuz from Iranian threats, signaling a greater openness to working with the United States to shore up maritime security in the region and acknowledging that its efforts to create a European-led task force have proved difficult.

Tensions between Britain and Iran have been rising since Iran seized a tanker flying the British flag in July. That followed Britain’s decision to impound an Iranian tanker near Gibraltar on suspicion that it violated a European Union embargo on the sale of oil to Syria.

But Britain at the time resisted the idea of joining its maritime forces with the United States, and instead described plans for a European-led mission in the area.

Britain’s decision to join the American-led security effort came after Prime Minister Boris Johnson took office at the end of last month. Mr. Johnson has made strengthening ties with the United States a priority, in part because a free-trade agreement with Washington is a centerpiece of his plans for Brexit.