The United States's top military officer says that despite an escalation in rhetoric between the U.S. and North Korea, he had not seen Pyongyang change its military posture.

"While the political space is clearly very charged right now, we haven't seen a change in the posture of North Korean forces and we watch that very closely," marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told a Senate armed services committee hearing for his reappointment on Tuesday.

"What we haven't seen is military activity that would be reflective of the charged political environment."

Dunford's comments come after reports from South Korea's Yonhap news agency Tuesday that North Korea appeared to have boosted defences on its east coast. This news came a day after North Korea said U.S. President Donald Trump had declared war and that it would shoot down U.S. bombers flying near the peninsula.

Yonhap suggested the reclusive North was in fact bolstering its defences by moving aircraft to its east coast and taking other measures after U.S. bombers flew close to the Korean Peninsula on the weekend.

The unverified Yonhap report said the United States appeared to have disclosed the flight route of the bombers intentionally because North Korea seemed to be unaware. South Korea's national intelligence service was unable to confirm the report immediately.

Dunford also said, "It would be an incredibly provocative thing for them to conduct a nuclear test in the Pacific as they have suggested, and I think the North Korean people would have to realize how serious that would be, not only for the United States but for the international community."

Threats and insults

Tensions have escalated since North Korea conducted its sixth and most powerful nuclear test on Sept. 3, but the rhetoric has reached a new level in recent days with leaders on both sides exchanging threats and insults.

Ri: 'This is clearly a declaration of war'1:36

North Korean Foreign Minister Ri Yong Ho said Trump's weekend Twitter comments, in which the U.S. leader said Ri and leader Kim Jong-un "won't be around much longer" if they acted on their threats, amounted to a declaration of war and that Pyongyang had the right to take countermeasures.

Trump continued the rhetoric Tuesday at a White House news conference. Asked about Ri's claim that the U.S. had declared war against North Korea, the president responded that if he has to take the military option, it will be "devastating for North Korea."

Trump said Kim is "saying things that should never be said."

War would have no winner, China says

Speaking in Beijing, Chinese foreign ministry spokesperson Lu Kang said war on the Korean Peninsula would have no winner.

"We hope the U.S. and North Korean politicians have sufficient political judgment to realize that resorting to military force will never be a viable way to resolve the peninsula issue and their own concerns," Lu told a daily news briefing.

"We also hope that both sides can realize that being bent on assertiveness and provoking each other will only increase the risk of conflict and reduce room for policy manoeuvres. War on the peninsula will have no winner."

Kim Jong-un and Donald Trump trade insults amid escalating U.S.- North Korea tensions2:13

Increasingly tough sanctions

The Trump administration on Tuesday announced new sanctions against North Korean banks, while Trump said it's time for all nations "to join forces to isolate to the North Korean menace."

The U.S. also imposed sanctions against 26 people, including some with ties to China, the U.S. Treasury Department's Office Of Foreign Assets Control Sanctions said on Tuesday. The U.S. sanctions target people in North Korea and some North Korean nationals in China, Russia, Libya and Dubai, according to a list posted on the agency's website.

Trump thanked the UN Security Council for its two recent "hard-hitting" resolutions on North Korea.

And he repeated praise he offered last week for China "breaking off all banking relationships with North Korea."

There has been no official confirmation from China of such a step. China is North Korea's most important trading partner.

Trump was speaking at a news conference Tuesday after meeting Spanish prime minister Mariano Rajoy.

North Korea Threats

U.S. President Donald Trump is seen in a North Korea propaganda video. (DPRK Today via Associated Press)

While repeatedly calling for dialogue to resolve the issue, China has also signed up for increasingly tough UN sanctions against North Korea.

China's fuel exports to North Korea fell in August, along with iron ore imports from the isolated nation, as trade slowed after the latest UN sanctions, but coal shipments resumed after a five-month hiatus, customs data showed on Tuesday.

U.S. Secretary of State Rex Tillerson will visit China this week to discuss North Korea as well as Trump's planned travel to the region. The State Department says Tillerson heads to Beijing on Thursday.

In Moscow, Russia's Foreign Ministry said it was working behind the scenes to find a political solution and that the usefulness of sanctions against North Korea was almost exhausted.

U.S. Defence Secretary James Mattis, speaking during a visit to India, said he appreciated global efforts to increase pressure on North Korea for its dangerous behaviour.

US North Korea

On Saturday, U.S. Air Force B-1B Lancer bombers escorted by fighter jets flew east of North Korea in a show of force. (Senior Airman John Linzmeier/U.S. Air Force/Associated Press)

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

The North says it needs its weapons programs to guard against U.S. invasion and regularly threatens to destroy the United States, South Korea and Japan.

With files from The Associated Press and CBC News