Anton Morozov, a member of the Russian lower house of parliament's international affairs committee, and two other Russian lawmakers visited Pyongyang on Oct. 2-6, Russia's RIA news agency reported.
"They are preparing for new tests of a long-range missile. They even gave us mathematical calculations that they believe prove that their missile can hit the west coast of the United States," RIA quoted Morozov as saying.
"As far as we understand, they intend to launch one more long-range missile in the near future. And in general, their mood is rather belligerent."
Tensions have risen in recent weeks over North Korea's nuclear weapons and missile programs as Pyongyang has test-fired several missiles and conducted what it said was a test explosion of a hydrogen bomb as it advances towards its goal of developing a nuclear-tipped missile capable of hitting the U.S. mainland.
Morozov's comments drove up the price of U.S. Treasury bonds, as investors, worried about the prospect of new North Korean missile tests, moved into assets the market views as a safe haven in times of uncertainty.
Reuters was not able to independently verify Morozov's account, and he did not specify which North Korean officials had given him the information about the planned test.
In Washington, a U.S. official said that there had been indications that North Korea could be preparing for a missile test on or around Oct. 10, the anniversary of the founding of the ruling Korean Workers Party and a day after the Columbus Day holiday in the United States.
The official, speaking on condition of anonymity, did not disclose the type of missile that could be tested and cautioned that North Korea in the past has not staged launches despite indications that it would.
A senior CIA analyst, speaking at a conference in Washington this week, said the North Korean government likely would stage some kind of provocation on Oct. 10 but did not elaborate on what form it might take.
"There is a clarity of purpose in what (North Korean leader) Kim Jong Un is doing. I don't think he's done," said Yong Suk Lee, the deputy assistant director of the CIA's Korea Mission Center, which was set up this year. "In fact, I told my own staff (that) October 10th is the Korean Workers Party founding day. That's Tuesday in North Korea, but Monday - the Columbus Day holiday - in the United States. So stand by your phones."
Morozov's delegation had "high-level" meetings in Pyongyang, RIA news agency said, citing the Russian embassy in the North Korean capital.
There has also been an exchange of tough rhetoric between Pyongyang and Washington.
U.S. President Donald Trump threatened to "totally destroy" North Korea if it threatens the United States. North Korean leader Kim Jong Un responded by calling Trump deranged and saying he would pay dearly for his threat.
Morozov is a member of the LDPR, a right-wing populist party. It casts itself as an opposition party, but hews close to the Kremlin line on matters of international affairs.
Describing meetings with North Korean officials, Morozov said they "displayed serious determination and bellicose rhetoric," RIA reported.
"The situation, of course, demands the swiftest intervention of all interested states, particularly those represented in the region, in order to prevent wide-scale military action," the agency quoted him as saying.
Russia has closer relations with Pyongyang than many other world powers, linked in part to Kim Il Sung, the founder of North Korea and the current leader's grand-father, having lived for a time in the Soviet Union.
Russian President Vladimir Putin has joined other world powers in condemning North Korea's weapons programme, but has taken a softer line than Western governments.
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