The US says it has begun deploying the first parts of a controversial missile defence system in South Korea, reports BBC.
The Terminal High-Altitude Area Defense system (Thaad) is designed to protect against threats from North Korea.
The move to begin installing the system comes a day after the North launched four ballistic missiles, breaking international sanctions.
But the US-South Korean arrangement has angered many on the Korean peninsula and around the region, including China.
Chinese foreign spokesman Geng Shuang said on Tuesday that China resolutely opposed its deployment.
South Korean media said operations to install a Thaad battery had begun on Monday, with some parts flown from the US to an air base near Seoul. A statement from the US military confirmed that the "first elements" of the system had been sent to South Korea.
South Korea's Yonhap news agency, quoting a military official, said the system could be operational "as early as April".
Many South Koreans believe the defence system will itself become a target, endangering people who live around the military sites.
Opponents have staged multiple demonstrations. In August last year, residents of the south-eastern Seongju region earmarked for Thaad had their heads shaved in a show of anger. China and Russia have both raised concerns over the deployment of the system, saying its radar could penetrate deep into their countries. Last year, China's foreign minister said the system went "far beyond" the defence needs of the Korean peninsula and directly affected China's "strategic security interests".
There has also been an outpouring of anger by Chinese consumers against the South Korean retailer Lotte, which allowed Thaad to be installed on one of its sites.
North Korea had threatened last week to fire missiles in response to the start of the Foal Eagle US-South Korean military exercises. The annual drills infuriate the North, which sees them as preparation for an invasion from the South. Then early on Monday, it fired four missiles from the Tongchang-ri region, near the border with China.
Three of the projectiles flew some 1,000km (620 miles) and fell into Japan's exclusive economic zone (EEZ). The exact type of missile is not yet clear but South Korean officials said they appeared to be an upgraded version of a Scud missile.
Admiral Harry Harris, commander of the US South Pacific Command, said the launch confirmed "the prudence" of the decision to deploy Thaad.
US President Donald Trump and Japan's Prime Minister Shinzo Abe have held phone discussions. The two leaders agreed that the launch was "a clear challenge to the region and international community". Abe added that the threat from North Korea had "entered a new phase".