"In a foreign country, U.S. citizens are subject to that country's laws, even if they differ from those in the United States," the State Department said.
The mother of an American citizen behind bars in the Philippines on drug charges fears for his life.
Orion Alfonso Hamre Tamayo, 26, was arrested May 20 after going to a local customs office to collect a parcel. That parcel, whose sender Tamayo says he doesn't know, contained 30 vials of liquid THC, which is the active ingredient in marijuana.
The government of President Rodrigo Duterte takes a hard-line approach toward drugs. According to official figures, more than 4,900 suspected drug dealers and users were killed in police operations between June 2016 and last September. The country's prisons are also overcrowded with conditions often being poor.
Tamayo has been charged with drug smuggling, according to the State Department.
His mother, Davina Stewart, is convinced her "happy-go-lucky" and "very soft-hearted" son is innocent.
"This is absolutely soul-crushing and heart-shattering," Stewart told NBC News from her home in Bloomington, Minnesota.
She described Tamayo as trusting, loyal and sometime naive.
"I believe his life is in danger," she added. "This has been a nightmare."
Stewart has spoken to her son by phone since his arrest. He is being held in a cell at the Philippine Drug Enforcement Agency headquarters in Quezon City, she says.
Tamayo, who is a dual American and Filipino citizen, has never had a run-in with the law, according to his mother.
In 2011, he moved to the Philippines at 18 to attend college. Tamayo loved the country's culture so much that he decided to stay after completing his mass media studies.
Despite being arrested, Tamayo told his mother not to worry.
"I think everything is going to be all right. It just takes a long time for due process is all," Tamayo said in a voicemail he left for Stewart. "I'm OK. I'm sorry you had to hear this."
The State Department said it was monitoring Tamayo's case and providing consular services.
"In a foreign country, U.S. citizens are subject to that country's laws, even if they differ from those in the United States," a State Department official said.
NBC News contacted the Philippine Drug Enforcement Agency for comment on Tamayo's case, but has yet to receive a response.
Human Rights Watch has condemned Duterte for what it described as a "murderous 'war on drugs.'"
The New York-based group added: "The exact number of fatalities is difficult to ascertain because the government has failed to disclose official documents about the 'drug war.'"
Raymund Narag spent nearly seven years inside a Filipino prison after being wrongly accused of murder following a 1995 brawl between fraternities at the University of the Philippines. He was eventually acquitted.
Narag, 44, is now an assistant professor at Southern Illinois University.
He said the average inmate in the Philippines spends 592 days in prison before getting a trial.
"Some people wait 16 years for a trial," added Narag, who is working to reform the country's prison system.