Their overseas sales plunged 14 percent to 527,605 units last month from the previous year's 615,418 units. Domestic sales declined 7.1 percent to 135,443 from 145,815, the data showed.

The carmakers' poor performance in May was mainly due to a low-growth trend in emerging countries and, in particular, lower demand from China, the world's biggest automobile market by sales volume, the companies said.

Last month, Hyundai and Kia continued to suffer declines in sales in China amid an ongoing row between Seoul and Beijing over the deployment of the Terminal High Altitude Area Defense system.

China has opposed the THAAD installation in South Korea as it claimed the battery system's powerful X-band radar system could spy on its military. Seoul has maintained the anti-missile system is aimed at countering missile threats from North Korea.

Sales by Hyundai and Kia fell 14 percent and 9.8 percent to 367,969 and 219,128 units last month, respectively, from a year earlier, the companies said.

As the country's two biggest carmakers expect slowing growth in advanced markets and uncertainties in emerging markets, they plan to launch models that are specific to the countries to boost sales.

As part of efforts to increase SUV sales, Hyundai will launch the Kona subcompact SUV this month in the domestic market.

The companies do not have any new, competitive SUV models to promote in the US market where SUV makers are benefiting from low gas prices and growing demand for the recreational vehicles. 

The three other carmakers also struggled last month as they also had no new models to attract consumers.

GM Korea's sales declined 17 percent from a year earlier to 43,085 autos last month. SsangYong Motor's fell 7.0 percent year-over-year to 12,349 vehicles. Renault Samsung's were down 15 percent to 20,517 units, the companies said.

In the January-May period, the five companies sold a combined 3.31 million autos, down 6.8 percent from 3.55 million units tallied a year earlier. Their domestic sales fell 1.7 percent year-on-year to 639,843 units and their overseas sales totaled 2.67 million, a 7.9 percent on-year decline, they said. (Yonhap)