He said on Thursday that the United States was “in very good shape” on N. Korea and that China's Xi, whom he met this month was working hard to help.

U.S. President Donald Trump praised Chinese efforts to rein in “the menace of North Korea” on Thursday, after North Korean state media warned the United States of a “super-mighty preemptive strike.”

Mr. Trump told a news conference “some very unusual moves have been made over the last two or three hours,” and that he was confident Chinese President Xi Jinping would “try very hard” to pressure Beijing's ally and neighbor North Korea over its nuclear and missile programs.

While Mr. Trump gave no indication of what the moves might be, U.S. officials told Reuters that the United States was aware of a higher-than-usual level of activity by Chinese bombers, signaling a possible heightened state of readiness. The officials played down concerns and left open a range of possible reasons.

Those possibilities included defensive exercises or Chinese concerns over North Korea. None of the officials, who spoke on condition of anonymity, suggested alarm or signaled that they knew the precise reason for such Chinese activity.

U.S. officials have been saying for weeks that North Korea could soon stage another nuclear bomb test, something both the United States and China have both warned against.

Mr. Trump has taken a hard line with North Korean leader Kim Jong Un, who has proceeded with nuclear and missile programmes in defiance of U.N. Security Council sanctions.

The United States and South Korea are technically still at war with North Korea because their 1950-53 conflict ended in a truce, not a peace treaty.

Tensions have risen sharply in recent months after North Korea conducted two nuclear weapons tests last year and carried out a steady stream of ballistic missile tests. Trump, who took office in January, has vowed to prevent North Korea from being able to hit the United States with a nuclear missile.

Mr. Trump has focused his efforts on North Korea on trying to persuade China to put more pressure on its ally and could view any unusual Chinese military movements as supportive of this.

He told a news conference with the visiting prime minister of Italy on Thursday that the United States was “in very good shape” on North Korea and that China's Xi, whom he met this month for a summit in Florida, was working hard to help.

“We dont know whether or not theyre able to do that, but I have absolute confidence that he will be trying very very hard.”

Mr. Trump repeated a past comment that he had told Mr. Xi in Florida that China would make much better deal on trade with the United States “if you get rid of this menace or do something about this menace of North Korea.”

Super-mighty premeptive strike

The Rodong Sinmun, the official newspaper of North Korea's ruling Workers' Party, struck an aggressive tone earlier on Thursday.

“In the case of our super-mighty preemptive strike being launched, it will completely and immediately wipe out not only U.S. imperialists' invasion forces in South Korea and its surrounding areas but the U.S. mainland and reduce them to ashes,” it said.

Reclusive North Korea regularly threatens to destroy Japan, South Korea and the United States and has shown no let-up in its belligerence after a failed missile test on Sunday, which followed a huge display of missiles at a parade in Pyongyang.

South Korean news agency Yonhap quoted an unnamed South Korean government source as saying that the U.S. Air Force had dispatched a nuclear sniffer aircraft on Thursday to the east of the Korean Peninsula in anticipation of a possible nuclear test.

The U.S. Defense Department does not comment on deployments of the WC-135 Constant Phoenix aircraft used to collect samples from the atmosphere to detect and analyse nuclear explosions.

The U.N. Security Council on Thursday condemned North Korea's latest failed missile test and demanded it not conduct any more nuclear tests.

South Korea's acting president, Hwang Kyo-ahn, at a meeting with top officials on Thursday, repeatedly called for the military and security ministries to maintain vigilance.

The South Korean defence ministry said U.S. and South Korean air forces were conducting an annual training exercise, codenamed Max Thunder, until April 28. North Korea routinely labels such exercises preparations for invasion.

“We are conducting a practical and more intensive exercise than ever,” South Korean pilot Colonel Lee Bum-chul told reporters. “Through this exercise, I am sure we can deter war and remove our enemy's intention to provoke us.”

William Perry, who served as U.S. defense secretary from 1994 to 1997 and negotiated with North Korea, said he did not believe Pyongyang was planning a surprise attack, despite the fiery rhetoric.

But he warned: “They are doing a lot of bluster and a lot of threats, and they might misplay that hand and blunder into a war.”