North Korea's missile sites have recently shown increased activities in the wake of the communist nation's threat to strike South Korea and the United States in response to their ongoing military drill involving nuclear-capable bombers, military sources in Seoul said Friday.

The latest move comes after the North Korean military on Tuesday put its strategic rocket units on the highest level of combat readiness, threatening to strike targets in South Korea as well as U.S. bases in Hawaii and Guam.

Following the U.S. military's unprecedented announcement that its nuclear-capable B-2 stealth bombers participated in the joint military drill in the South, North Korean leader Kim Jong-un on Friday ordered the country's strategic rocket forces to assume

combat ready status in a fresh threat to hit U.S. bases with its long-range missiles.

Amid heightened tensions, South Korean and U.S. forces have focused their intelligence efforts on monitoring the North's missile sites, which have shown brisk activities.

"North Korea's launch sites to fire off mid- and long-range missiles have recently shown increased movement of vehicles and forces," the official said, asking for anonymity. "We are closely watching possibilities of missile launches."

Another source said several vehicles were seen moving to the Tongchang-ri missile site on the western coast, in what seemed like preparations for testing its long-range missiles.

Seoul's defense ministry saw the latest move as a "follow-up measure" to the North Korean military's announcement of the heightened level of combat readiness, but it did not elaborate on "a rocket preparation plan" signed in the North.

Tensions on the peninsula have risen after the North launched a long-range rocket in December 2012 and conducted its third nuclear test last month, which drew new rounds of U.N. sanctions on the isolated state.

After the North announced it had voided the Armistice Agreement and cut the military hotline connecting two Koreas, concerns rose over unprovoked attacks by the recalcitrant neighbor, which launched deadly attacks near the western maritime border in 2010.

Although Pyongyang has routinely denounced past annual drills in the South, Seoul and Washington have taken its bellicose rhetoric more seriously this year as the North has even threatened an "all-out battle" and "pre-emptive nuclear strike" against the

U.S. mainland, which are unusually intense threats.

Outside watchers, however, consider Pyongyang's latest remarks to be aimed at strengthening control domestically and building military credentials, raising doubts over whether it has the technology to build an intercontinental ballistic missile capable of hitting the U.S. mainland.(Yonhap News)