China has signaled that it will impose international and unilateral sanctions on North Korea if Pyongyang follows through with announced plans to set off a fourth underground nuclear test.

Meanwhile, North Korea has stepped up anti-U.S. rhetoric, issuing an official government statement that referred to President Obama as a "wicked black monkey."

The over-the-top verbal attacks are viewed by U.S. intelligence analysts as a response to international pressure not to conduct nuclear or missile tests that would further destabilize the region.

The indication China will impose sanctions on North Korea for another nuclear test appeared in a pro-Beijing news report from Hong Kong April 30 that quoted a Chinese professor.

"If the DPRK should indeed conduct another nuclear test, China will definitely be prepared to play a leading role in joining other nations to endorse another U.N. Security Council resolution to impose sanctions on the DPRK in a collective international manner," said Shi Yinhong, a professor at the Communist Party’s People’s University in Beijing. DPRK is the acronym for Democratic People’s Republic of Korea, the North’s formal name.

Air Force Gen. Herbert "Hawk" Carlisle, commander of the U.S. Air Forces in the Pacific, said this week that the North Korean nuclear testing activity is increasing tensions. He also said development of a new KN-08 road-mobile intercontinental missile and recent launches had increased the danger of a conflict.

"That string for tension in the Korean peninsula is truly as tight as it’s ever been," Carlisle said Monday at the Center for Strategic and International Studies. "And it appears to be getting worse to us."

Shi warned during the Phoenix TV interview that China is prepared to impose unilateral sanctions on North Korea if actions by Pyongyang undermine stability on the peninsula.

"If the action of the DPRK should bring about serious threat to the peace and security of the Korean Peninsula, or if it should seriously violate the denuclearization principle by conducting nuclear tests or long range missile tests, then China will impose its own unilateral sanctions on the DPRK—apart from joining the international community in imposing collective sanctions," he said.

The comments are being viewed by U.S. officials as the first clear indicator that China is attempting to use its diplomatic and economic leverage to prevent a fourth underground nuclear test.

China frequently uses what appear to be academics to send diplomatic messages that are in fact reflections of official Communist Party policy.

China in the past has abstained from or opposed votes on the imposition of U.N. sanctions on North Korea.

Joseph DeTrani, former special envoy for North Korean nuclear talks during the George W. Bush administration, said the Chinese report of possible sanctions is significant.

"If China were to impose unilateral sanctions on North Korea if they had a fourth nuclear test, it would be a powerful message to the leadership in Pyongyang that they have gone too far; that their escalation of tension over the past two years has created a dangerous international environment that China will not tolerate," said DeTrani, now head of the Intelligence and National Security Alliance. "For the region and the world, this would be a welcome response from China, a neighbor and ally of North Korea."

U.S. intelligence agencies are closely monitoring the Punggye-ri Nuclear Test Site in the far northeastern part of North Korea amid increased activity indicating preparations are underway for a nuclear test blast.

The website 38 North reported May 2 that satellite images revealed "a significant increase in activity in the west portal area of North Korea’s Punggye-ri Nuclear Test Site related to the excavation of a new test tunnel begun one year ago."

"At the same time, there appears to be a drawdown in activity in the south portal area, believed to be the likely location of North Korea’s next nuclear test."

Excavation activity at nuclear testing sites is part of underground testing. A tunnel is dug and a nuclear device is placed at the bottom, with wires leading out of the tunnel to a monitoring station.

North Korea for its past three nuclear tests in 2006, 2009, and last year conducted the tests within a month of a formal announcement that a test was planned.

However, it has been more than 30 days since North Korea announced on March 30 that it planned an unspecified "new form" of nuclear test. The March 30 statement said tests of medium- and long-range nuclear delivery systems were planned and the "new form" nuclear test was not ruled out.

Some intelligence analysts believe the new test could be North Korea’s first test of a small warhead capable of being delivered on a missile, or a combined long-range missile and test blast.

Intelligence agencies were on high alert for the test during the recent visit to South Korea and Asia by Obama.

Then on April 29 North Korea appeared to extend a deadline for the next nuclear test. A Foreign Ministry statement said the nuclear test had not been ruled out and that there was "no statute of limitations" for one.

Also, a nuclear test without a missile launch would mark a departure from Pyongyang’s past practice. Prior to each of the three nuclear tests, North Korea fired missiles in periods ranging from three months to two months before the test.

The delay in a nuclear test from the predicted 30-day window could be the result of pressure from China not to carry out the test, U.S. officials said.

Some analysts believe the increase in harsh anti-U.S. rhetoric coincides with the change in North Korea’s posture of readying missile and nuclear tests.

The April 29 statement accused the United States of launching a campaign to overthrow the Pyongyang regime by force. The statement referred to the Obama administration as the "gang of Obama"—terminology normally used to describe South Korean anti-communists and conservatives.

A statement a day earlier from the National Defense Commission, the ultimate power organ in North Korea, described Obama as a "wolf" who had his teeth in South Korea.

The use of racial slurs and sexual insults are a sign of increased hostility by North Korea that began with the ratcheting up of verbal attacks targeting Obama and South Korea’s conservative President Park Geun-hye late last month.

The official KCNA news agency on April 27 called Park a "capricious whore" and Obama’s "pimp."

Then on May 2 a KCNA editorial blamed Park for the Sewol ferry disaster that left hundreds of school children dead. It included the racist epithet to describe Obama.

According to an English translation, a paragraph of the editorial described Obama’s visit to South Korea thus: "Park made wastewater-like reckless remarks slandering the DPRK's line on simultaneously developing two fronts after inviting her American master reminiscent of a wicked black monkey to visit South Korea on April 25."

Of Park, the editorial said: "All Koreans are spitting on her as she is resorting to whorish and disgusting political prostitution only after leaving her soul or chastity violated at such old age of over 60."

On Monday, South Korean Foreign Minister Yun Byung-se issued a new warning that North Korea may carry out a "fresh nuclear tests" or missile launch. Yun said during a visit to New York that the international community "should slap strong sanctions on Pyongyang, enough to put the kibosh on the issue."

Meanwhile, another semi-official Chinese government spokesman predicted this week that North Korea will conduct a nuclear test.

Yang Xiyu, a researcher of the China Institute of International Affairs, told the Communist Youth League newspaper May 5 that he believes North Korea will conduct another test for both technical and political reasons.

"The DPRK hopes to use nuclear deterrent to make the United States and the ROK unwilling to attack it," Yang stated.

To further reiterate Beijing’s message, Shi, the People’s University professor, also said China would not impose further sanctions if Pyongyang agrees not to conduct another nuclear test blast.

The Phoenix TV report also quoted Chinese military expert Peng Guangqian, also addressed a fourth nuclear test as "speculative reporting" by the news media.

"One is just not certain about its validity," Peng said, reflecting the Chinese military’s close ties to the North Korean military.

"And in fact, the DPRK issue is in essence a U.S. issue—one that has been caused by erroneous U.S. policy towards the DPRK," he said. "The DPRK issue is not at all an issue of China; it would be unfair for one to see it as a China issue. Those who attribute the DPRK issue to China are actually harboring ulterior motives."