The incursions by Chinese and Russian warplanes into South Korea's air defense zone this week appeared aimed at testing security ties between Seoul, Washington and Tokyo, analysts said Wednesday.

On Tuesday, two Russian bombers and two Chinese bombers breached Korea's Air Defense Identification Zone, before a Russian early warning and control aircraft committed an unprecedented violation of the country's airspace.

The KADIZ breaches came amid concerns that the spat between Seoul and Tokyo over Japan's wartime forced labor and its recent export restrictions could hurt their security cooperation, and by extension, their trilateral military collaboration with the United States.

They also coincided with a visit to Seoul by US National Security Advisor John Bolton, during which he was expected to stress the importance of America's three-way security cooperation with its two core Asian allies, South Korea and Japan.

"There are possibilities that the KADIZ breaches were designed to check the robustness of security cooperation between South Korea and Japan, and among South Korea, the US and Japan," said Nam Chang-hee, a professor of international politics at Inha University.

"(China and Russia) might have apparently sought to see how South Korea and Japan would respond, though it remains uncertain whether they timed such flights to coincide with Bolton's visit to Seoul," he added.

A country declares an air defense identification zone for the early identification of foreign military aircraft approaching its airspace. The zone is not part of its airspace, but a foreign warplane nearing the zone should make a prior notification in line with international customs.

It is not the first time that Chinese and Russian warplanes have violated the KADIZ. Analysts presumed that in past breaches, they might have gleaned information about South Korea's military radar frequencies needed for potential jamming operations in case of a conflict.

But this time, the joint maneuvers by China and Russia into the KADIZ attracted greater attention, as there have been worries that the tensions between South Korea and Japan could reopen cracks in the US' alliance network, which Washington has long touted as a key to regional stability.

The spat between the neighbors, starting with the issue of forced labor stemming from Japan's 1910-45 colonization of the peninsula, has been spilling over into the economic domain with Tokyo's July 4 enforcement of restrictions on exports to South Korea of three key industrial materials used in semiconductors and displays.

Talk of its possible impact on the two countries' security cooperation has also emerged, as Seoul has hinted that it could review whether to renew a military intelligence-sharing pact with Tokyo seen as a crucial platform for trilateral defense cooperation involving Washington.

The intrusions into the KADIZ also came as China, currently in a trade conflict with the US, has been pushing to project military power farther into the Pacific, where Washington is maintaining a strong naval presence through freedom-of-navigation operations and other means.

"The US is seeking to beef up its naval operations, particularly in the South China Sea, and given their strategic calculations, one cannot rule out the possibility that the KADIZ intrusions came in connection with China's efforts to build power presence beyond its shores," said professor Nam.

Some observers argue that the KADIZ intrusions could also be related to the US initiative to form an international military coalition to safeguard freedom of navigation in the strategically vital Strait of Hormuz off Iran. China and Russia are known to oppose the US military move in the waters. (Yonhap)