Two Chinese destroyers and three helicopters are engaged in maritime exercises off the Hong Kong waters in a show of force just days after Beijing's fighter jets had a close encounter with a US plane over the South China Sea. Though the Chinese military has called the exercise "routine", it gains significance since it is taking place ahead of the 20th anniversary of Britain's handover of Hong Kong to China.

The latest drills are thought to be directed at the US, which recently protested against a "dangerously" close interception of its surveillance plane by two Chinese jets. Washington, on 27 May, said the encounter – the second within a week – was "unsafe and unprofessional". The intercept was conducted just 240kms east of Hong Kong.

The two Chinese warships - Qinzhou and Huizhou - are reported to be sailing towards Hong Kong's Victoria Harbour from its Stonecutters Island navy base.

The Chinese defence ministry said the exercises by the destroyers and three reconnaissance-and-rescue helicopters are part of Beijing's routine monitoring of activities in the area. The ministry's statement added that such drills are necessary to look for suspicious activities in case there is an emergency situation. China's indigenous Beidou satellite navigation system and other military communication systems were also being tested during the joint air and sea patrol.

The vessels and aircraft are part of the Hong Kong Garrison of the Chinese People's Liberation Army.

"The message is clear and loud. Hong Kong, as part of China, is also under the threat of military reconnaissance and surveillance by other countries," Ni Lexiong, a Shanghai-based military expert, told the South China Morning Post, adding that the latest Chinese moves are clearly to challenge US' assertions to conduct freedom of navigation in the region.

The drills are taking place just as the quasi-sovereign city-state, where calls for complete democracy are constantly growing, is preparing to mark the handover anniversary on 1 July. Chinese President Xi Jinping is also expected to make a three-day visit to Hong Kong beginning on 29 July.