The U.S. switched on a controversial missile defense system in South Korea Tuesday, just hours after it flew a pair of strategic bombers over the region — a move described by North Korea as “reckless” and pushing the area “closer to nuclear war.”

Pyongyang is said to have viewed the incident – where two supersonic B-1B Lancer bombers flew from Guam to take part in military exercises with the South Korean and Japanese air forces – as “a nuclear bomb–dropping drill” at a time when Donald Trump and “other U.S. warmongers are crying out for making a pre-emptive nuclear strike” on the North.

“The reckless military provocation is pushing the situation on the Korean Peninsula closer to the brink of nuclear war,” the state-run KCNA news agency reported.

South Korea’s government said the purpose of the mission was to deter provocations by the North and to test readiness against another potential nuclear test.

Heightening tensions in a region already on high alert was the announcement Tuesday by the U.S. military that its Terminal High-Altitude Area Defense system (THAAD) was now “operational and has the ability to intercept North Korean missiles” — a move originally expected to come at the end of 2017. The news comes less than a week after the U.S. began installing the system on a former golf course in the town of Seongju, in the southeastern part of the country.

THAAD is designed to destroy short- and medium-range ballistic missiles during their final phase of flight, but it has only “reached initial intercept capability,” a U.S. official said, as the system will be upgraded later this year with additional hardware.

Just hours after the system came online, China’s foreign ministry called on the U.S. to halt its deployment: “China’s position is clear-cut and firm,” foreign ministry spokesman Geng Shuang told reporters at a regular press briefing. “We oppose the deployment of the THAAD system and urge relevant sides to immediately stop the deployment. We will firmly take necessary measures to uphold our interests.”

Part of those “necessary measures” may include cyberattacks. On Monday U.S. cybersecurity firm FireEye reported that state-sponsored hackers from China attempted to compromise the security of one of the groups involved in installing the $1 billion missile array.

The attack was likely an information-gathering mission, as China has consistently opposed the deployment of THAAD in South Korea, claiming that its radar system can be modified to allow the U.S. military to spy on its own military operations.

South Korea confirmed there was a cyberattack on its systems last month that came from China, though the statement stopped short of saying the attack had targeted THAAD specifically. China strongly denied the accusation, saying it opposes any form of cyberattack. “This position is consistent, clear, and serious,” the foreign ministry said in a statement.

As well as opposition from China, the THAAD system has been opposed by locals, with several hundred angry Seongju residents turning out last week to protest as the first components were delivered to the site. Locals believe that the location of THAAD puts them at risk of attack.

The controversial hardware is also likely to influence the outcome of next week’s presidential election, with Moon Jae-in, the moderate liberal front-runner, suggesting he could renegotiate the terms of the deployment.

Last week Trump appeared to be doing something similar when he said that South Korea should pay the estimated $1 billion for the anti-missile defense system. However, on Sunday, White House national security chief Lt. Gen. H.R. McMaster walked back that claim in a phone call to his South Korean counterpart, saying that the U.S. would put up the money.