China says it has sent its Russian-made Sukhoi Su-35 advanced fighter jets for training under “real combat” conditions in the South China Sea, a potential show of force against the controversial presence of the United States military in the disputed region.

According to a strongly-worded statement released by the China’s People’s Liberation Army (PLA) on Wednesday, the military drill was a “pragmatic action” aimed at fulfilling “its mission in the new era.”

The statement further said that the training was expected to boost pilots’ capabilities “under long-distance or high-sea conditions,” but did not indicate the exact number of fighter jets that took part in the exercise.

The jets are dubbed in China “the killer in the sky” for being highly-maneuverable and equipped with phased array radar and steerable thrusters.

China and Russia inked a deal for the shipment of 24 Su-35 aircraft for a sum of two billion dollars in November 2015. The first batch, comprising of four jets, was reportedly delivered in late 2016 while another 10 aircraft were handed over in late 2017.  The remaining 10 warplanes are reportedly to be delivered in 2018.

‘Military drill in response to US’

According to Xu Guangyu, a retired PLA general and senior adviser at the China Arms Control and Disarmament Association, the air force drill was launched in response to “provocation by the US,” an apparent reference to an incident in which US Navy missile destroyer USS Hopper came too close to a South China Sea island — disputed between China and the Philippines — on January 17.

On January 20, the Chinese Foreign Ministry, while reporting the date of the incident, strongly denounced the US move and vowed to take “necessary measures” to protect China’s sovereignty.

Xu added that the military exercise further meant to show that the “China-Russia military cooperation is solid, mutually benefiting and reliable.”

China is involved in territorial disputes in the South China Sea, where several countries, including Vietnam, Taiwan, Malaysia, and the Philippines, have overlapping claims. The US, which is an extra-regional country, has been taking the side of China’s rival claimants in the regional disputes.

Washington has so far sent several of its warships to the South China Sea to protect what it calls “freedom of navigation” there, but Beijing often accuses Washington of interfering in regional issues and deliberately stirring up tensions.