Chinese president Xi Jinping set off on a visit to Russia on Monday stressing the grave threat a US anti-missile system in South Korea poses to both Chinese and Russian interests.
Beijing has repeatedly stated its opposition to the US Terminal High Altitude Area Defence (Thaad) anti-missile system and has called for its deployment to be stopped, and the missiles already installed to be removed.
The system's powerful radar can probe deep into its territory, China says, undermining its security and a regional balance while doing nothing to stop North Korea in its relentless pursuit of nuclear weapons and the missiles to deliver them.
In separate calls with Mr Xi and Japanese prime minister Shinzo Abe on Monday, US president Donald Trump discussed tensions on the Korean peninsula.
Mr Trump and his top aides have done little to hide their irritation over what they see as the reluctance by China, North Korea's main economic partner, to tighten the screws on Pyongyang over its nuclear and missile programmes.
During the phone call, Mr Xi told Mr Trump that Sino-US relations have been hit by "negative factors" following days of US actions that have vexed Beijing.
China has been angered by US arms sales to rival Taiwan, American sanctions against a Chinese bank over its dealings with North Korea and, most recently, the sailing of a US destroyer within the territorial seas limit of a Chinese-claimed island in the South China Sea.
The US State Department also gave China a dismal grade last week in a new human trafficking report.
According to state media, Mr Xi told Mr Trump in their call that Beijing expects Washington to continue managing relations on the basis of the "one China" principle that rules out formal contacts with Taiwan, which China claims as its own territory.
The Chinese leader, however, insisted that the two sides should follow the "consensus" reached at Mar-a-Lago in April and said the two presidents agreed to meet at the Group of 20 summit in Germany this week.
Ahead of his visit to Russia, Mr Xi said: "The US deployment of an advanced anti-missile system in South Korea gravely harms the strategic security interests of China, Russia and other countries in the region."
"Beijing and Moscow are steadfastly opposed to the THAAD deployment and seriously suggest that relevant countries stop and cancel the installation," China's state Xinhua news agency cited him as saying.
The US and South Korea say the Thaad is solely aimed at defending the South from North Korea.
But China and Russia have maintained close communication and co-ordination on the issue and held very similar views on it, Mr Xi said in an interview with Russian media.
China and Russia would take "necessary measures", either together or independently, to protect their interests, Mr Xi said, without elaborating, according to Xinhua.
Mr Xi also said Beijing and Moscow should work together to boost trade and increase investment and financial co-operation, Xinhua reported.
After his state visit to Moscow, Mr Xi will travel to Germany to attend a G20 summit.
Relations between China and South Korea have been strained by the THAAD deployment though both sides have struck a more conciliatory tone since President Moon Jae-in took office in South Korea May.
* with additional reporting from Agence France-Presse and Associated Press
