ISTANBUL, Aug 26, 2025 (BSS/AFP) - A Russian athlete who went missing during Istanbul's annual cross-continental Bosphorus swimming race did not leave the water according to data from his tracking chip, the authorities said Monday.

The swimmer, 30-year-old Nikolai Svechnikov, had started the 6.5-kilometre (4-mile) race between Asia and Europe that took place on Sunday morning with more than 2,800 participants from 81 countries, officials said.

With the Bosphorus Strait -- one of the world's busiest shipping lanes -- temporarily closed to traffic, the swimmers were racing from Kanlica neighbourhood on the Asian side to Kurucesme on the European side.

"Russian athlete Nikolai Svechnikov ... went missing during the race," said the Turkish Olympic Committee, which organised the race.

In a statement, the Istanbul governor's office said Svechnikov, a professional swimming coach, "had not made it to the finish line".

"Security camera footage and the chip attached to the swimmer's foot showed he began the race but did not get out at the finish line," it said.

There was still no trace of him late Monday, with Istanbul prosecutors opening an investigation into his disappearance, the private DHA news agency reported.

Russia's consulate general in Istanbul said it was in contact with Turkish authorities about the situation and confirmed Svechnikov's relatives had been informed.

According to the official race website, swimmers have a time limit of two hours to complete the course, with the coast guard picking up anyone who does not finish within that time.

The Olympic Committee said the race began at 10:04 am (0704 GMT) with the last swimmer entering the water at 10:54 am.

After the two-hour race limit finished, an initial check found that three swimmers were missing, but this was later reduced to a single athlete, it said.

"This information was immediately shared with the coast guard command, the maritime police department, and the Istanbul governor's office, and search and rescue operations were ... immediately initiated," it said.

"We are working in full cooperation with the coast guard and police units to shed light on the incident."