BEIJING - Russian-Chinese defence cooperation is a bilateral issue of two sovereign countries in which the United States is not allowed to meddle, Chinese Defence Ministry Spokesman Ren Guoqiang said on Thursday.

Last week, the United States sanctioned a key Chinese military research unit for buying Russian-made aircraft and an S-400 air defence system.

"China and Russia enjoy the relations of a comprehensive strategic partnership, defence cooperation is normal between two sovereign states," Ren told reporters, answering the question on whether the US sanctions could affect Moscow-Beijing defence relations.

According to the spokesman, such cooperation is aimed at protection of the two states' interests in which "the United States has no right to interfere."

All Russian-Chinese projects in defence are progressing as planned, the spokesman added.

The United States has been actively using sanctions under its 2017 Countering America's Adversaries Through Sanctions Act (CAATSA) to promote US interests worldwide.

Both Russia and China have fallen under the scope of US restrictions. In particular, a number of Russian entities and individuals have been sanctioned over Moscow's alleged meddling in US elections, which the Russian authorities denied. China has been targeted by US trade sanctions, including restrictions on investment and tariffs on Chinese imports.

Chinese blames US for deterioration of bilateral military relations

China believes that the ties between the armed forces of China and the United States were damaged due to the wrongful actions of Washington, Chinese Defence Ministry spokesman Ren Guoqiang said on Thursday.

Both countries have been engaged in a trade dispute for several months now. Most recently, the United States imposed sanctions on China for purchasing Russian-made weapons systems and S-400 air Defence systems. Beijing also recalled the People’s Liberation Army (PLA) Navy commander Vice Adm. Shen Jinlong’s from his US visit and postponed planned China-US talks in Beijing.

“China has always taken a positive approach to maintaining contacts between the US and Chinese military at all levels. The responsibility for the damage caused to the relations between the two countries lies entirely with the US side,” Ren said.

The ministry spokesman stressed that the current problem had arisen because of the erroneous actions of the United States. He therefore suggested that Washington immediately correct this mistake and lift the sanctions to eliminate obstacles to the normal development of relations between the armies of the two countries.

“The Chinese side attaches great importance to the development of relations between the armed forces of the two countries, but will never accept foreign interference and threats. China’s decisions that are based on ensuring its national interests are unshakable,” Ren added. China wants the United States to take concrete measures to create favorable conditions for exchanges and cooperation between the armies of the two countries.

China slams US B-52s fly over disputed seas as 'provocative'

AFP adds: The Chinese defence ministry on Thursday denounced flyovers by US B-52 bombers over the South China Sea and East China Sea as "provocative" actions amid soaring tensions between the two global powers.

The Pentagon said Wednesday the heavy bombers had taken part in a combined operation with Japan over the East China Sea and had flown through international airspace over the South China Sea a day before.

"Regarding the provocative actions of US military aircraft in the South China Sea, we are always resolutely opposed to them, and will continue to take necessary measures in order to strongly handle (this issue)," Chinese defence ministry spokesman Ren Guoqiang told a monthly news briefing.

China has claimed large swaths of the strategic waterway and built up a series of islands and maritime features, turning them into military facilities.

Brunei, Malaysia, the Philippines, Taiwan and Vietnam have competing claims to the region, and an international maritime tribunal ruled in 2016 that China's claims have no legal basis.

Pentagon spokesman Lieutenant Colonel Dave Eastburn said this week's flights were part of "regularly scheduled operations."

The United States rejects China's territorial claims and routinely says the military will "continue to fly, sail and operate wherever international law allows at times and places of our choosing."

Washington this week enacted new tariffs against China covering another $200 billion of its imports while it last week sanctioned a Chinese military organisation for buying Russian weapons.

China has reacted angrily, and this week scrapped a US warship's planned port visit to Hong Kong and cancelled a meeting between the head of the Chinese navy and his American counterpart.

US Defense Secretary Jim Mattis said he was not concerned the US transit flights would increase tensions with China.

"If it was 20 years ago and they have not militarised those features there, it would have just been another bomber on its way to Diego Garcia or whatever," he told Pentagon reporters, referring to the US military base in the Indian Ocean.

"So there's nothing out of the ordinary about it, nor about our ships sailing through there."

The Pentagon chief went on to say there is no "fundamental shift in anything."

"We're just going through one of those periodic points where we've got to learn to manage our differences," he said.