WASHINGTON, July 20 (Reuters) - North Korea remained silent on Thursday about a U.S. soldier who two days earlier split from a tour group and made a dash across the heavily fortified border, landing Washington in a new diplomatic quagmire amid an already tense military standoff.

U.S. officials said Pyongyang has not responded to communication from the American military about the soldier, Private Travis T. King. North Korea's state media, which in the past reported on the detention of U.S. nationals, have also not commented on the incident so far.

State Department spokesperson Matthew Miller told a briefing the Pentagon had "reached out" to counterparts in North Korea's Korean People's Army about King, but added: "My understanding is that those communications have not yet been answered."

The incident comes at a time of heightened tension on the Korean peninsula. The North has been pressing on with ballistic missiles tests, the latest timed for the arrival in the South of a U.S. nuclear-armed ballistic missile submarine for the first time since the 1980s.

Last week, the North launched its newest solid-fuel intercontinental ballistic missile which it said had the longest flight time ever, a test that experts described as a "remarkable" success.

King was on a civilian tour of the Panmunjom truce village when he suddenly dashed across the Military Demarcation Line that has separated the two Korea since the Korean War ended in 1953 with an armistice.

King had gone through legal troubles while stationed in South Korea, serving detention before he was being escorted to the airport this week to fly back to the United States, two U.S. officials said.

Under circumstances that remained unclear, he turned around after passing through security at the gate and fled. Later, he was with a group of about 40 on a tour of the Joint Security Area (JSA) on the military border.