Relations between Kyiv and Beijing, a key Russian ally, are strained. Ukraine and the West accuse China of enabling the Russian invasion through trade and of supplying technology, including for deadly drone attacks.

Ukraine also says dozens of Chinese citizens have been recruited by Russia's army and sent to fight.

The SBU "detained two citizens of the People's Republic of China in Kyiv who were attempting to illegally export secret documentation on the Ukrainian RK-360MC Neptune missile system to China," the agency said in a statement.

The son is a 24-year-old former student of a technical university in Kyiv, it said, adding that the father, who lives in China, had travelled to Ukraine to coordinate his son's "espionage activities".

If convicted, they face up to 15 years in prison on espionage charges.

Kyiv used the anti-ship Neptune missile to destroy the flagship of Russia's Black Sea Fleet, Moskva, in the early days of the more than three-year war.

In April, Ukraine's military captured two Chinese nationals and said it had details of dozens more who were fighting for the Russian army.

Beijing rejected the allegations that its citizens were being recruited en masse to fight for Russia as groundless and irresponsible.

Moscow and Beijing struck a "no limits" partnership on the eve of Russia's February 2022 invasion, and have since deepened political, military and economic cooperation.