"The U.S. administration and Congress seem to be in solidarity on taking a hard-line mode on North Korea".
Satellite analysis now indicates increased activity at two key sites - the Samundong missile research facility and the Sohae rocket launch centre.
North Korean leader Kim Jong Un, who recently held a second round of talks with US President Donald Trump in Vietnam, is the most prominent candidate and is running for re-election in his Pyongyang district.
"North Korea is just emulating all other Communist states", he said.
The Trump administration has taken a harder-line approach to denuclearizing North Korea since the summit in Vietnam last month, raising doubts about whether the two sides will reach a deal on the centerpiece of President Trump's foreign policy. However, Trump told reporters that like the Singapore summit, Kim had committed to not conducting nuclear or ballistic missile tests.
"I would be very disappointed if that were happening. It's a very early report", Trump said.
Mr Bolton added: "It's possible that North Korea will go back and rethink the position they came in with and come back to talk to the president about the big deal".
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Images of Samundong taken on February 22 showed cars and trucks at the site, as well as rail cars and cranes at a yard, U.S. news outlet NPR has reported. Raddatz asked Bolton Sunday on "This Week".
Trump said on Friday he would be disappointed if Pyongyang were to resume weapons testing and reiterated his belief in his good relationship with Kim despite the collapse of the summit.
In the interview, Bolton had also said that Washington is watching North Korean activities "unblinkingly".
"Diplomacy is still very much alive", Biegun said, although he offered no specifics on when new talks might be held and did not say whether any talks had taken place since the summit, which collapsed over differences on USA demands for Pyongyang's denuclearization and North Korea's demand for sanctions relief.
"The election will strikingly manifest the fixed will of our people to firmly trust and uphold to the last Supreme Leader Kim Jong Un despite storm and stress", the ruling party's official daily said in a commentary on Sunday. "So that's why, action for action, nearly inevitably in the past three administration has worked to North Korea's benefit".
But in the Vietnamese capital of Hanoi, the US side back-pedalled and called for a comprehensive deal, Moon said.
United Nations experts are investigating possible violations of UN sanctions on North Korea in about 20 countries, from alleged clandestine nuclear procurement in China to arms brokering in Syria and military cooperation with Iran, Libya and Sudan.