WASHINGTON 芒鈧珺次元官网网址� If China was hoping for a concession from the U.S. after recently suspending coal imports from cash-poor North Korea, it got the opposite.

The U.S. is starting to deploy a missile defence system in South Korea which the allies say is needed to defend against North Korea. But China and North Korea view it as a threat.

That adds to strains in the high-stakes relationship between Washington and Beijing weeks into Donald Trump's presidency. And it could further complicate co-operation between the two world powers in combating something they both oppose 芒鈧珺次元官网网址� North Korea's nuclear program.

The U.S. military said Tuesday it has started bringing in equipment for the deployment of the Terminal High-Altitude Area Defence system, or THAAD. It says the system is designed to intercept and destroy short and medium-range ballistic missiles during the last part of their flights.

The announcement came after North Korea on Monday launched four ballistic missiles into ocean off Japan 芒鈧珺次元官网网址� a show of force in response to annual U.S.-South Korean military drills. The allies say they are routine but Pyongyang views them as rehearsal for an invasion.

China is concerned that THAAD has powerful radar that could track Chinese missiles and weaken its deterrence against the United States. Beijing said Tuesday it will take "necessary measures" to protect itself and warning that the U.S. and South Korea should be prepared to bear the consequences.

China has already exerted pressure on some South Korean commercial interests, but it remains unclear how the deployment will impact ties with Washington, which insists China is not targeted by THAAD. U.S.-based experts say that Chinese concerns are exaggerated, if not entirely unwarranted.

"The question is whether China having expressed their grievances will be prepared to let this pass or will let it erode their relationship with South Korea and a meaningful capacity for co-operation with the United States on North Korea," said Jonathan Pollack of the Brookings Institution think-tank .

China proposed Wednesday that North Korea could suspend its nuclear and missile activities in exchange for a halt in joint military drills conducted by the U.S. and South Korea.

Foreign Minister Wang Yi said in Beijing that the "suspension-for-suspension can help us break out of the security dilemma and bring the parties back to the negotiating table."

North Korea is one of the main national security challenges facing the Trump administration. It may already have the capability to strike South Korea and Japan 芒鈧珺次元官网网址� and the tens of thousands of American forces there 芒鈧珺次元官网网址� with nuclear weapons, and could pose the same threat to the U.S. mainland within years.

For China, the North's nuclear program poses a less direct challenge, but it could destabilize Northeast Asia and incentivize Japan, a rival of China's to acquire nuclear weapons of their own. And it could encourage the U.S. to strengthen its military posture in the region with systems like THAAD.

In a sign of growing impatience with North Korean leader Kim Jong Un, China announced last month it was suspending coal imports from North Korea in adherence to U.N. sanctions passed in November after the isolated nation's most powerful nuclear test explosion to date.

But Beijing remains reluctant to exert too much pressure on its traditional ally, fearing that a regime collapse would lead to a unified Korea allied with Washington at its border.

Joel Wit, a former State Department official and specialist on North Korea, said that the main factor in the continuing disconnect between the U.S. and China on dealing with Pyongyang is rooted not in differences over missile defence, but in a stalemate over how to approach the problem.

"The U.S. is always talking about increasing pressure, and taking military steps to protect its allies and itself," he said. "But the Chinese are always talking about need to reach out and talk to the North Koreans."

State Department acting spokesman Mark Toner made clear Tuesday that the U.S. administration is not seeking direct engagement with a provocative Pyongyang. When Secretary of State Rex Tillerson makes his first visit to the region next week, with stops in Tokyo, Seoul and Beijing, he'll be seeking a "strategic co-ordination" to address the escalating threat posed by North Korea.

"Right now we are focused on sanctions, and implementing those sanctions to the fullest extent possible, but we're looking at other possibilities as well," Toner said, without elaborating.

The THAAD deployment will hang over Tillerson's meetings with the Chinese.

Missile expert John Schilling said Beijing's concern is that the system's radar could provide detailed tracking of Chinese long-range missiles and that could be used to alert U.S. missile defences.

"This is somewhat exaggerated as a threat, but the full capabilities of the THAAD system are classified so it can't be entirely discounted," he said, adding that China could counter that by modifying their missiles with multiple warheads or decoys, a process that it has likely begun already.

"We have to consider whether this leads in the direction of an arms race between United States and China," Schilling said, "which I don't think anybody wants."

Matthew Pennington, The Associated Press