SEOUL, South Korea — The United States announced Wednesday that it was speeding the deployment of an advanced missile defense system to Guam in the next few weeks, two years ahead of schedule in what the Pentagon said was “a precautionary move” to protect American naval and air forces from the threat of a North Korean missile attack.

The system — called Thaad, for Terminal High Altitude Area Defense — was scheduled for deployment around 2015.

The decision to deploy it now was the latest in a series of steps intended to deter the North from either military action or new missile tests and came only hours after the latest North Korean provocation, with officials blocking South Koreans from crossing the border to enter a jointly operated industrial park.

The North had threatened the move in reaction to taunts from the South Korean news media that the North Korean leader, Kim Jong-un, had cut hot lines and other communications across the border, but did not want to risk one of his most precious sources of hard currency. The border has been sealed before, but the move against the operations at the industrial park that employs roughly 53,000 North Koreans raised doubt about the future of the last remaining major symbol of inter-Korean cooperation.