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PYONGYANG — North Korean state media warned on Tuesday of a nuclear attack on the United States at any sign of American aggression as a U.S. Navy strike group steamed toward the western Pacific.

U.S. President Donald Trump, who has urged China to do more to rein in its impoverished neighbor, said in a tweet that North Korea was "looking for trouble" and the United States would "solve the problem" with or without China's help.

Tension has escalated sharply on the Korean peninsula, with talk of military action by the United States gaining traction following its strikes last week against Syria and amid concerns the reclusive North may soon conduct a sixth nuclear test.

North Korea's official Rodong Sinmun newspaper said the country was prepared to respond to any aggression by the United States.

"Our revolutionary strong army is keenly watching every move by enemy elements with our nuclear sight focused on the U.S. invasionary bases not only in South Korea and the Pacific operation theater but also in the U.S. mainland," it said.

North Korean Leader Kim Jong Un guides the Korean People's Army Tank Crews' Competition 2017 in an undated photo released on April 1. KCNA via Reuters

South Korea's acting President Hwang Kyo-ahn warned of "greater provocations" by North Korea and ordered the military to intensify monitoring and to ensure close communication with the United States.

"It is possible the North may wage greater provocations such as a nuclear test timed with various anniversaries including the Supreme People's Assembly," said Hwang, acting leader since former president Park Geun-hye was removed amid a graft scandal.

Related: Trump's Options for North Korea Include Placing Nukes in South Korea

Trump said in a tweet that a trade deal between China and the United States would be "far better for them if they solved the North Korea problem".

"If China decides to help, that would be great," he said. "If not, we will solve the problem without them!"

Trump and his Chinese counterpart Xi Jinping met in Florida last week and Trump pressed Xi to do more to rein in North Korea.

The North convened a Supreme People's Assembly session on Tuesday, one of its twice-yearly sessions attended by leader Kim Jong Un, and reported a successful national budget execution and personnel appointments, official KCNA news agency said.

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There was no mention of its nuclear weapons program or being under threat from the United States, according to KCNA.

South Korean officials took pains to quell talk in social media of an impending security crisis or outbreak of war.

"We'd like to ask for precaution so as not to get blinded by exaggerated assessment about the security situation on the Korean peninsula," Defense Ministry spokesman Moon Sang-kyun said.

A fast patrol boat from South Korea's First Naval Fleet guards a group of boats catching fish in the East Sea bordering North Korea on Tuesday. South Korea Navy / EPA

Saturday is the 105th anniversary of the birth of Kim Il Sung, the country's founding father and grandfather of current ruler, Kim Jong Un.

A military parade is expected in the North's capital Pyongyang to mark the day. North Korea often also marks important anniversaries with tests of its nuclear or missile capabilities in breach of U.N. Security Council resolutions.

Men and women in colorful outfits were singing and dancing on the streets of Pyongyang, illuminated by better lighting than seen in previous years, apparently practising for the parade.

Syrian President Bashar al-Assad sent a message of congratulations to mark the event, lambasting "big powers" for their "expansionist" policy.

"The friendly two countries are celebrating this anniversary and, at the same time, conducting a war against big powers' wild ambition to subject all countries to their expansionist and dominationist policy and deprive them of their rights to self-determination," the North's KCNA news agency quoted the message as saying.

The North's foreign ministry, in a statement carried by KCNA, said the U.S. Navy strike group's approach showed Washington's "reckless moves for invading had reached a serious phase".

"We never beg for peace but we will take the toughest counteraction against the provocateurs in order to defend ourselves by powerful force of arms and keep to the road chosen by ourselves," an unidentified ministry spokesman said.

North Korea and the rich, democratic South are technically still at war because their 1950-1953 conflict ended in a truce, not a peace treaty. The North regularly threatens to destroy the South and its main ally, the United States.

Russian worries

North Korea is emerging as one of the most pressing foreign policy problems facing the Trump administration.

The North has conducted five nuclear tests, two of them last year, and is working to develop nuclear-tipped missiles that can reach the United States.

The Trump administration is reviewing its policy towards North Korea and has said all options are on the table, including military strikes, but U.S. officials said non-military action appeared to be at the top of the list.

Russia's foreign ministry, in a statement ahead of a visit by U.S. Secretary of State Rex Tillerson, said it was concerned about many aspects of U.S. foreign policy, particularly on North Korea.

U.S. Marine Corps participate in a joint defense exercise against a possible attack from North Korea, in Pohang, South Korea on Tuesday. Jeon Heon-Kyun / EPA

"We are really worried about what Washington has in mind for North Korea after it hinted at the possibility of a unilateral military scenario," the ministry said.

"It's important to understand how that would tally with collective obligations on de-nuclearising the Korean peninsula, something that is underpinned in U.N. Security Council resolutions."

Russia condemned U.S. cruise missile strikes on a Syrian air base on Friday, calling them an illegal attack on a sovereign state.

The U.S. Navy strike group Carl Vinson was diverted from port calls to Australia and would move toward the western Pacific Ocean near the Korean peninsula as a show of force, a U.S. official told Reuters over the weekend.

U.S. officials said the strike group would take more than a week to reach waters near the Korean peninsula.

Sailors conduct flight operations on the aircraft carrier USS Carl Vinson flight deck, in the South China Sea on April 8. Matt Brown / U.S. Navy via Reuters

China and South Korea agreed on Monday to impose tougher sanctions on North Korea if it carried out nuclear or long-range missile tests, a senior official in Seoul said.

On Tuesday, a fleet of North Korean cargo ships was heading home, most of the vessels fully laden, after China ordered its trading companies to return the coal to curb the trade, sources with direct knowledge of the trade said.

The order was given on April 7, just as Trump and Xi were set for the summit where they agreed the North Korean nuclear advances had reached a "very serious stage", Tillerson said.

Following repeated missile tests that drew international criticism, China banned all imports of North Korean coal on Feb. 26, cutting off the country's most important export product.

The North is seen ready to conduct its sixth nuclear test at any time, with movements detected by satellite at its Punggye-ri nuclear test site.