US Secretary of State Marco Rubio has demanded that Panama make "immediate changes" to what he calls the "influence and control" of China over the Panama Canal,
BBC reports.
America's top diplomat said Panama must act or the US would take necessary measures to protect its rights under a treaty between the two countries.
The warning follows President Donald Trump's vow to retake the canal and a meeting between Rubio and Jose Raul Mulino, Panama's conservative president, in Panama City on Sunday.
The two men appeared to emerge from their two-hour meeting with different interpretations.
Mulino told reporters he did not see a serious threat of US military force to seize the canal, saying he had proposed technical-level talks with the US to address Mr Trump's concerns about Chinese influence.
However, Trump's vow to retake the canal has sparked a significant backlash in Panama. Protesters in Panama City on Friday burned effigies of Trump and Rubio.
Riot police moved in on another crowd of demonstrators, firing tear gas and wrestling people away. The clashes were small-scale, but the resistance to the US president's stance is widely felt.
On Thursday, Mulino said the issue of the canal's ownership would not be up for discussion with Rubio.
"I cannot negotiate or even open a negotiation process about the canal. It's sealed, the canal belongs to Panama," he said.
Mr Trump's comments about the canal included an unfounded claim that Chinese soldiers are operating it. He also said American ships were unfairly charged more than others, despite the fact such a practice would be unlawful under treaty agreements.
The waterway is in fact owned and operated by the Panamanian government, under a neutrality treaty signed with the US decades ago. However, Chinese companies have invested heavily in ports and terminals near the canal. A Hong Kong based company runs two of the five ports close to its entrances.
But President Trump's muscular approach - even refusing to rule out military action to take the canal - has aroused a strongly patriotic reaction in the small strategic nation.
"It's ridiculous," says Panama City resident Mari, who asked not to have her surname published.
"There's a treaty that he has to respect, and there's nothing in the treaty that says that we cannot have ports run by the Chinese," she told the BBC, pointing out that there is Chinese investment in American ports and cities.
