An Iranian appeals court has upheld the convictions of a prominent Iranian-American father and son accused of collaborating with the United States, their lawyer said Monday, posing a new source of tension in the increasing hostility between the countries.

Iran’s incarceration of the defendants, Baquer Namazi and his son, Siamak, who were convicted last year and sentenced to 10-year terms, has been repeatedly cited by President Trump in his denunciations of the Iranian authorities.

News that their appeal had been rejected came amid numerous signs of the downward spiral in the relations between Iran and the United States. The most notable is the Trump administration’s assertions that Iran is violating the 2015 nuclear agreement reached under President Barack Obama. The administration also has infuriated Iran by imposing new sanctions on the country in recent weeks.

Jared Genser, a Washington-based lawyer for the Namazis, said in a statement that their family was informed on Sunday that a Tehran appellate court had upheld the convictions five days earlier. The precise reason for the court’s lag in conveying the decision was not clear, Mr. Genser said. “No written appeals decisions have been provided to the family,” the lawyer said.