WASHINGTON (Reuters) - The U.S. State Department said on Tuesday it was aware of reports of the detention of a U.S. citizen held in Iran for more than six months, amid heightened tensions between the countries following the reimposition of U.S. sanctions.

The New York Times reported on Monday that Michael R. White, a 46-year-old U.S. Navy veteran, was seized while visiting Iran and has been held in jail since July on unspecified charges.

His mother, Joanne White, told the New York Times her son had visited Iran “five or six times” to meet his Iranian girlfriend. He had bought a ticket to return from Iran, but never boarded his flight on July 27.

Asked about the reports, a U.S. State Department spokesman said: “We are aware of reports of the detention of a U.S. citizen in Iran.”

The spokesman declined to provide additional information, citing privacy considerations.

Iranian officials have not reacted to the reports, and were not immediately available for comment.

A former detainee in Iran, Ivar Farhadi, told the London-based IranWire website he had spoken to White when they were both at Vakilabad Prison in the city of Mashhad in northeastern Iran.

The New York Times quoted White’s mother as saying her son, a California resident, suffers acute asthma and had undergone chemotherapy and radiation treatment for a neck tumor.

Tension between Iran and the United States has risen significantly since last May, when U.S. President Donald Trump withdrew from an international nuclear deal with Tehran and reimposed U.S. sanctions that had been lifted after a 2015 accord.

Several Americans have been detained in Iran in recent years and Trump warned in 2017 that Tehran would face “new and serious consequences” unless all unjustly held U.S. citizens were freed.

Former FBI agent Robert Levinson disappeared while visiting Iran’s Kish Island in 2007. In 2016, during President Barack Obama’s administration, senior U.S. officials said they believed Levinson had died in captivity.

On Jan. 12, a State Department spokesman said that characterization was inaccurate, adding: “The U.S. government continues to reiterate its call on the Government of the Islamic Republic of Iran to locate Mr. Levinson and return him home to his family.”

Iranian officials have repeatedly denied knowledge of Levinson’s disappearance or whereabouts.

In October 2015, Siamak Namazi, a businessman in his mid-40s with dual U.S.-Iranian citizenship, was detained as he visited family in Tehran. His 82-year-old father, Baquer Namazi, was also arrested in February 2016 and later convicted of espionage charges which he denied.

Xiyue Wang, a Chinese-born U.S. citizen and graduate student from Princeton University, was arrested in Iran in 2016. He was sentenced to 10 years in jail on spying charges that he denied.