The activity appears to involve some modifications around one of the tunnel entrances to an underground test area.
The officials said it is not yet clear if the activity indicates a sixth nuclear test is imminent, but noted there is concern that North Korea could set off a test during Wednesday's visit to Washington by top Chinese diplomats and military officials.
US officials have known that the site is ready to conduct an underground test for some time.
Two senior US officials with direct knowledge also told CNN that military options for North Korea have recently been updated, and will be presented to President Donald Trump for a decision to act if there is a nuclear test.
However, there is no indication whether any military response would actually happen.
US officials told CNN that if a sixth nuclear test by North Korea were to occur, it would be clear that the existing pressure by China on North Korea is not working. However, other senior US officials said the administration's policy hinges on pressuring North Korea through China.
Trump has often cited China, North Korea's longtime ally, as a key player in US efforts to rein in North Korea's quest to develop long-range nuclear missiles. But the US President took to Twitter on Tuesday to offer a grim assessment of China's role in restraining North Korea: "While I greatly appreciate the efforts of President Xi & China to help with North Korea, it has not worked out. At least I know China tried!"
Multiple US officials told CNN they are checking on what the President meant in his latest tweet about how China's efforts in North Korea have "not worked out."
One Trump administration official said bluntly they didn't know what the President was referencing when asked what the tweet meant. Another said there was no meeting earlier Tuesday that they believe could have spurred Trump's comment, and pushed back on the idea that the tweet could be a hint of some forthcoming diplomatic or military action.
White House press secretary Sean Spicer painted a rosier picture on Tuesday, telling reporters that the US is continuing to work with China to apply pressure on North Korea and said the US has seen "positive movement" with China.
The US will "continue to work with them and others to put the appropriate pressure on North Korea to change the behavior of this regime," Spicer said.
Asked whether Trump would still consider meeting with Kim Jong Un given the right conditions, Spicer said: "Clearly we're moving further away, not closer, to those conditions being met."
The Chinese have moved a bit toward restraining North Korea, but not enough for administration officials' taste -- and there is reason to be skeptical as to how much more they will move. But the US officials said it will be at least a year before they can evaluate whether the North Koreans are feeling pressure because of China.
In a recent exchange with Sen. Lindsay Graham on Capitol Hill, Defense Secretary James Mattis took an unusually specific stand on US military policy.
Graham asked: "Is it the policy of the Trump administration to deny North Korea the capability of building an (intercontinental ballistic missile) that can hit the American homeland with a nuclear weapon on top? Is that the policy?"
Mattis answered simply: "Yes."
Mattis and Secretary of State Rex Tillerson are hosting their Chinese counterparts on Wednesday for the US-China Diplomatic and Security Dialogue in Washington. At the bilateral talks tomorrow, officials say the US is looking to use trade as a weapon with China to put more pressure on North Korea.
Most estimates suggest that Kim's unpredictable regime is between three and five years away from achieving its nuclear ambitions.