John Miller, 63, and Cui Guanghai, 43, were detained in Belgrade in late April on a US warrant accusing them of "conspiracy, smuggling, and violations of the Arms Export Control Act" by allegedly attempting to obtain and transfer sensitive US military technology to China, according to the US Department of Justice.

Extradition proceedings were launched at Washington's request and, after spending several weeks in detention, the two were placed under house arrest with electronic monitoring.

But in mid-August, "the electronic monitoring system triggered an alarm", the High Court told AFP on Tuesday.

"When officials went to their registered addresses, it was established that the individuals whose extradition is sought were not present... As a result, they are no longer under monitoring and are now fugitives," the court said in a written statement.

The court has issued a fresh arrest warrant for the pair.

Asked for comment, the Chinese foreign ministry told AFP that it was "not aware of the situation," while the British Foreign, Commonwealth and Development Office (FCDO) said it was "in touch with local authorities" and "supporting the family" of a detained British national.

"Any ongoing court proceedings are a matter for the Serbian authorities," the FCDO added in a written statement.

AFP requested details on the case from Serbian authorities, but they did not respond.

The reasons for the two men's presence in Belgrade remain unclear, although Serbia and China are linked by numerous commercial agreements and close diplomatic ties.

In 2024, Belgrade was one of three cities visited by Chinese President Xi Jinping during his European tour.

Serbian President Aleksandar Vucic was also among the few European leaders in Beijing in early September to mark the 80th anniversary of the end of World War II and victory over Japan.