BEIJING — The Chinese government is ratcheting up pressure on South Korea over its plans to deploy an American missile defense system, with the state-controlled news media urging the public to boycott South Korean retail products and threatening diplomatic and even military repercussions.

China’s latest pronouncements follow months of not-so-subtle punitive measures that have already taken a toll on the South Korean economy, including an unofficial ban on Korean television shows and pop stars. The campaign risks a backlash in South Korea even as Beijing’s relations with North Korea have also grown strained — a sign of how recent advances in the North’s nuclear program have put China in a bind and are upsetting the regional security balance.

On Thursday, South Korea and the United States began talks in Seoul to finalize details of the deployment of the so-called Terminal High-Altitude Area Defense System, or Thaad, according to the South’s Foreign Ministry. Both countries say the system’s purpose is to defend the South against North Korea’s growing missile and nuclear threat, but China has objected strongly to the system, which it sees as an American attempt to encircle it.

No date has been set for the system’s deployment, but the Pentagon said on Wednesday that Defense Secretary Jim Mattis wanted it in place “as soon as feasible.” Military experts said the United States could use C-17 transport aircraft to quickly move the system’s truck-mounted launchers, interceptors, radar, fire control units and support equipment to South Korea.