American citizen believed to have been in Slovenia has been missing for weeks

An American man is believed to have gone missing in Slovenia while vacationing in June. The Department of State is working with police in Europe on the continuing search.

Jonathan Reid Luskin from Wisconsin, also known as "Jon," is believed to have left Austria for Slovenia after his mother went back home to the U.S. after their family vacation.

Luskin, 25, had been living and teaching in Hong Kong and left on June 11 for a summer vacation after the academic school year, his mother Susan Stone told ABC News. She said he met with the family in Europe.

On the morning of the June 22, Stone said she left for the U.S. and Luskin was on his way from Vienna, Austria, to Ljubljana, Slovenia, by train. She said that was the last time she saw her son.

Later that day, Stone said Luskin left a message on WhatsApp for one of his friends who lives in Hong Kong saying he had arrived in Slovenia and intended to go hiking on the weekend in Slovenia's Triglav National Park.

"I sent him a message on June 25 that he didn’t respond to," Stone told ABC News.

"He had an account with Citibank in Hong Kong and there have been no withdrawals since the middle of June," she said. "He has an 'Amex' card here in the States and there have been no activities on either one," she said, referring to an abbreviation of American Express.

After Stone hadn't heard from her son for weeks, Stone said she decided to report her son missing to U.S. State Department, Senator Tammy Baldwin’s office, and the U.S. Embassies in Slovenia, Croatia and Greece on July 20.

Slovenian police were informed on July 25 that a U.S. citizen from Wisconsin named Jonathan Reid Luskin was missing, a Slovenia police spokesperson told ABC News, confirming that Luskin was last seen in Vienna on the morning of June 22 when he took the train to Ljubljana.

Police are treating the case as high-profile and have informed the public about it; Luskin’s has been published in every national newspaper, the spokesperson added. They are also checking Luskin’s phone cell phone records and WhatsApp calls.

Through Interpol, Slovenian police are cooperating with police in Croatia, Austria, Greece, the U.S. and China, the spokesperson added.

The U.S. Department of State said it is also working with local authorities to find him.

"We are aware of reports of a U.S. citizen missing in Slovenia," an official with State Department told ABC News. "The U.S. Department of State and our embassies and consulates abroad have no greater responsibility than the protection of U.S. citizens overseas."

Luskin had further travel plans after Slovenia, according to his mother. She said he had planned to continue travel in Europe to Croatia and Greece before heading to Japan, then back to Hong Kong in early August.

But, there has been no evidence that Luskin traveled to any of those destinations, Stone said.

She believes her son entered the Triglav National Park in Slovenia for his hike on the morning of June 23, but he has not been seen or in contact with anyone since.

"Hiking would be something that he loves to do," Stone told ABC News. "I just want him to be okay."

ABC News' Dragana Jovanovic contributed reporting from Belgrade, Serbia, and ABC News' Conor Finnegan contributed reporting from Washington.