Over 200 reports of natural gas odors plagued Busan and neighboring Ulsan over the weekend, but the source of the odor has not yet been identified, authorities said Monday.

According to the Busan Metropolitan Government, the first report of a smell from a suspected gas leak came in at around 5:30 p.m. on Thursday from an area near Haeundae beach. More than 160 similar reports of a gas odor were received within two hours on the same day, mainly from Jung-dong in Haeundae-gu and Yongho-dong and Daeyeon-dong in Nam-gu.

Although Busan City immediately dispatched employees to check the gas pipes of the city, it found no gas leak.

City authorities on Sunday collected samples from four gas cargo trucks and shipping vessels that passed by Gwangan Bridge for analysis. But Busan’s metropolitan public health and environment research center said it found no evidence of a leak there.

Residents in Ulsan also complained about a suspicious gas odor shortly afterward.

The Ulsan Fire Department said Monday that it received 25 calls complaining of severe gas odors from Saturday afternoon.

Calls were mainly made from Sinjeong-dong, Yaeum-dong and Seonam-dong in Nam-gu, all within 5 kilometers of the Ulsan Petrochemical Complex.

The fire department dispatched on Saturday nine fire trucks and firefighters to the plants to find gas leaks and check for gas levels in the air, but still found nothing extraordinary.

With reports on the unidentified gas odor continuing, citizens raised concern that it may be early signs of earthquakes or malfunctions at a nuclear power plant in Gori near Busan.

Experts, however, said no signs of earthquakes or malfunctions at the nuclear plant had been detected in Busan or Ulsan.

“It is likely that a mass gas leak in Haeundae-gu in Busan or some other area could have wafted across the city via the wind,” said professor Oh Jae-hoh of Environmental Sciences and Technology from Pukyong National University.

On Sunday, a photo of an aerial shot of Haeundae beach went viral on social media. The picture showed a swarm of ants moving in a line on the beach. Some netizens raised suspicions that such a swarm of ants might correlate with the spread of the gas odor, but experts said that the ants might just be moving in that manner to look for food.

By Kim Da-sol (ddd@heraldcorp.com)