The acquittal was eventually overturned, and a Turkish court ruled last week to combine the two charges for a hearing to start in May.
"The specious charges against Kavala, his ongoing detention, and the continuing delays in the conclusion of his trial, including through the merger of cases against him, undermine respect for the rule of law and democracy," the US State Department said in a statement on Wednesday.
Turkey rejects call
In response, the Turkish Foreign Ministry told the US to respect the independence of its courts.
"Turkey is a state of law. No country or person can give orders to Turkish courts about legal proceedings," it said. The US "must respect" legal proceedings against Kavala "by independent courts."
The ministry added that it was "inconsistent and unprincipled" for the US to issue such demands while refusing to extradite US-based cleric Fethullah Gulen, who Ankara blames for the failed 2016 coup.