China continued moving tanks and armored vehicles and flying flights near North Korea this week as part of a military buildup in the northeastern part of the country that U.S. officials say is related to the crisis with North Korea.

The Obama administration, meanwhile, sought to play down the Chinese military buildup along the border with Beijing’s fraternal communist ally despite the growing danger of conflict following unprecedented threats by Pyongyang to attack the United States and South Korea with nuclear weapons.

According to U.S. officials with access to intelligence reports, both intelligence and Internet reports from the region over the past week revealed the modest military movements in the border region that began in mid-March and are continuing.

The buildup appears linked to North Korea’s March 30 announcement that it is in a "state of war" with South Korea after the United Nations imposed a new round of sanctions following the North’s Feb. 12 nuclear test and because of ongoing large-scale joint U.S.-South Korea military exercises.

The People’s Liberation Army (PLA) troop and tank movements were reported in Daqing, located in northeastern Heilongjiang Province, and in the border city of Shenyang, in Liaoning Province.

Officials said one key military unit involved in the mobilization is the 190th Mechanized Infantry Brigade based in Benxi, Liaoning Province. The brigade is believed to be the PLA’s frontline combat unit that would respond to any regional conflict or refugee flows. Troops and tank movements also were reported in Dandong, in Liaoning Province.

Fighter jets were reported flying in larger numbers in Fucheng, Hebei Province, and in Zhangwu, in Liaoning Province, and Changchun, in Jilin Province.

One of China’s Russian-made Su-27 jets crashed on Sunday in Rongcheng, a city directly across the Yellow Sea from the Korean peninsula. The accident may have been part of the increased warplane activity related to the military mobilization, officials said.

The PLA movements were first reported Monday by the Free Beacon.

The buildup likely serves two goals, the officials said. One is to bolster border security in case a conflict sends large numbers of refugees from the impoverished state into China.

Additionally, the troop buildup is a signal to Pyongyang that China will abide by its defense commitment to North Korea in the event of renewed conflict.

China’s military maintains a mutual defense treaty with North Korea. The last time Chinese troops defended North Korea was during the Korean War.

U.S. officials also said there were signs of increased movement inside North Korea, specifically movement of road-mobile missile systems. One official said activity was seen at the long-range missile launch complex at Tongchang-ri on the west coast.

Pentagon press secretary George Little was asked about possible North Korean missile launches and said test flights were possible.

"We can't rule out the possibility, obviously, that they may conduct some kind of tests or engage in some kind of provocative behavior that would cause problems," he said. "We hope that doesn't happen, but if history's any guide, it could. So we really need to be ready to respond, and that's our goal."

Asked about the Chinese military buildup near the border, Little did not deny reports of the mobilization but referred reporters to the Chinese military for comment.

"I have seen those reports," Little said of news reports of the Chinese border buildup. "But I would refer you to the Chinese military or the Chinese government for comment."

Little said he was unaware of any communication between the Pentagon and the Chinese military regarding the Korean situation.

Two U.S. missile defense warships, the destroyers USS Decatur and the USS McCain, were deployed in the western Pacific to deal with any North Korean missile threats, he said. A large sea-based x-band missile defense radar also could be moved closer to Korea, he said.

"Missile defense is an important priority for us in the Asia-Pacific and elsewhere, and we are postured to protect our allies and our own interests in this region and other regions of the world," Little told reporters at the Pentagon.

Chinese military and civilian spokesmen in Beijing made no mention of the northeast troop buildup in remarks to reporters at two briefings on Tuesday. The officials called for calm.

Chinese Defense Ministry spokesman Yang Yujun described the current situation on the Korean peninsula as "extremely complicated and sensitive." He said all sides should seek to ease tension and maintain regional peace and stability.

Chinese Foreign Ministry spokesman Hong Lei told reporters Tuesday that "we regret" Pyongyang’s announcement that it will restart a five-megawatt nuclear reactor.

Asked if recent Chinese military exercises could increase tensions, Hong said, "We believe that war on the Korean Peninsula would not serve the interest of any party. The parties concerned have a common interest in and a joint responsibility for maintaining peace and stability on the Korean Peninsula and in Northeast Asia at large. We hope that the parties concerned will act in the overall interest, remain calm, show restraint, resume dialogue and negotiations, improve relations, promote the denuclearization of the Korean Peninsula, and maintain enduring peace and stability in Northeast Asia."

However, a senior Chinese military official said Tuesday that the PLA should strengthen combat readiness to ensure victory in wars.

Gen. Fan Changlong, vice chairman of the Central Military Commission (CMC), made the comments during a visit to PLA troops in eastern China's Jiangsu, Fujian, and Zhejiang provinces, the state-run news outlet China.org.cn reported.

"Military officers and soldiers must be absolutely loyal, pure and reliable and firmly follow the directions of the Central Committee of the CPC, Central Military Commission and chairman Xi [Jinping]," Fan said.

Additionally, North Korea’s government announced it would step up nuclear weapons development by adopting what Pyongyang called a "new strategic line" that seeks to bolster both economic and nuclear development together.

The government issued a decree that sought to place the position of nuclear weapons for self-defense as a higher national priority.

One step is to restart the Yongbyon reactor that had been reportedly destroyed by North Korea in 2007 under international pressure.

State Department spokeswoman Victoria Nuland told reporters on Monday that the U.S. government does not recognize North Korea as a nuclear weapons state.

Gen. James Thurman, commander of U.S. forces in South Korea, told ABC News that the situation on the divided peninsula was as "volatile" and "dangerous" as he has seen in two years as commander.

South Korea’s Defense Ministry announced on Monday that it has adopted new contingency plans called "active deterrence" that would permit the military to carry out preemptive attacks against the North in the face of an imminent nuclear or missile strike.

One sign that relations had not reached a tipping point toward conflict despite the growing tensions on the peninsula is the fact that the North-South special economic zone at Kaesong remains open with South Korean workers traveling to the zone without disruption.

South Korea’s Daily NK newspaper, known for contacts inside North Korea, reported on Tuesday that a source in the country said orders for the entire country to go to a war footing were issued March 26, but only two days of lectures, rallies, and meetings for soldiers and civilians were held.

The North Korean government since then appears to have backed off the war preparations.

"They had the PSM (People’s Safety Ministry; the North Korean police force), people’s unit heads, and NSA (National Security Agency) all out there in meetings telling people they needed to stay vigilant, but people responded indifferently," the source told the newspaper. "It was because the measures had been going on for months; nobody had any further interest in them."