The U.S. military began deploying an advanced defensive missile system to South Korea on Monday, a long-awaited move that was agreed upon with the government in Seoul last summer and follows a series of provocations by North Korea.

The Terminal High Altitude Area Defense (THAAD) system does not use warheads and is designed to shoot down enemy ballistic missiles like the ones launched by Pyongyang into the Sea of Japan on Sunday. The deployment comes despite protests from Chinese officials, who have said repeatedly over the last year that it is threat to Chinese security.

“Continued provocative actions by North Korea, to include yesterday’s launch of multiple missiles, only confirm the prudence of our alliance decision last year to deploy THAAD to South Korea,” said Navy Adm. Harry Harris, the chief of U.S. Pacific Command. “We will resolutely honor our alliance commitments to South Korea and stand ready to defend ourselves, the American homeland, and our allies.”

The deployment was announced as North Korean state media reported that the four missiles Pyongyang launched Sunday were practice for a North Korean attack on U.S. military bases in Japan. Three of the missiles traveled about 600 miles over North Korea and the Sea of Japan before landing inside of Japan’s exclusive economic zone. The fourth landed just outside the EEZ.

North Korea also is believed to have thousands of artillery cannons aimed at its southern neighbor, a particular concern for Seoul because the city of 10 million is within range, 35 miles from North Korea’s border.

Each THAAD battery includes at least six truck-mounted launchers that carry up to eight missiles each. U.S. military officials stress that it provides strictly defensive capabilities, but it also possesses powerful radar that Chinese officials are concerned will be used to observe Chinese airspace.

“We firmly oppose the deployment of THAAD,” said Geng Shuang, a spokesperson for China’s foreign ministry at a daily press briefing in Beijing on Tuesday. “We will resolutely take necessary measures to defend our security interests.”

“All the consequences from this will be borne by the U.S. and the ROK,” he added. “We strongly urge relevant parties to stop deploying it and not travel down the wrong path.”

South Korea’s left-wing opposition parties also sharply criticized the government’s “unilateral, rushed” move to deploy the THAAD battery ahead of schedule.

The government “has completely disregarded public calls and demands from political circles with regard to the THAAD deployment,” said Youn Kwan-suk, the spokesman for the Democratic Party.

“From the clandestine push for the deployment to the process of making the final decision on the installation, the government has unilaterally pressed ahead with it without any consultations with citizens,” he said, according to the Yonhap news agency.

Several key details of the deployment have not yet been finalized and the land where the battery will be housed is not yet ready, opposition politicians said. But South Korean defense officials said they would keep the equipment at the U.S. air base at Osan until the site was ready.

When Seoul and Washington agreed last year on the THAAD unit, they were aiming for deployment by the end of July, although it had been expedited and was expected to arrive between June and August.

With the arrival of the first components, they could have it up and running sooner. “The deployment could be completed within one or two months, and it can be operational as early as April,” a South Korean military official told reporters in Seoul.

Conservative politicians said that North Korea’s latest missile launches underscored the urgent need for THAAD.

“We welcome the right decision to deploy THAAD when North Korea threatens the security of the Republic of Korea and Northeast Asia by firing as many as four ballistic missiles,” said Kim Sung-won, the spokesman for the ruling Liberty Korea Party.

The timing is politically sensitive because the president, Park Geun-hye, is likely to find out this week whether she will be impeached. The Constitutional Court is expected to issue its decision as soon as Friday on whether to uphold a parliamentary motion to force Park out of office over a corruption scandal.

Moon Jae-in, the leading liberal candidate for the presidency, has said he would respect agreements between governments, but has also suggested that he might ask parliament to revisit the THAAD deployment.

The United States and South Korea began discussing the deployment of THAAD more than a year ago, and reached an agreement to do so in July. Then-president Barrack Obama reaffirmed U.S. commitment to the deployment in October, saying Washington and Seoul have worked together to prepare for any threat. He specifically cited THAAD as an example, while calling it “a purely defensive system to deter and defend against North Korean threats.”

Defense Secretary Jim Mattis made his first trip as Pentagon chief to Seoul and Tokyo in January, and said then that he anticipated THAAD would arrive in South Korea this year.

“THAAD is for defenses of our ally’s people, of our troops who are committed to their defenses,” Mattis said.

The Washington Post’s Anna Fifield contributed to this report.