"Washington thinks it is highly likely that those activities are a part of North Korea's operations to close down the nuclear facility," the source told the paper.
It said the US images had been obtained on Monday and shared with South Korea.
South Korea's National Intelligence Service said it was aware of the activity but did not say what it might mean.
"There were some unusual movements there and we are now following and analysing them," a spokesman told AFP.
|A file satellite image of the Yongbyon nuclear |
facility [EPA/DIGITAL GLOBE]
South Korea's Yonhap news agency cited an unnamed intelligence official as saying that the Yongbyon reactor was still in operation, but there was a high possibility that the movement could be linked to a shutdown.
The report comes after the North missed a Saturday deadline to halt work at the reactor and allow UN inspectors to verify and seal the facility under a six-nation agreement signed in Beijing in February.
Pyongyang has not released any statement since the deadline passed.
However, it said last week that honouring its pledge was contingent on the release of $25m frozen in a separate financial dispute after the US blacklisted a Macau-based bank where North Korea had accounts.
The funds were allegedly used in money laundering and counterfeiting.
The US says the money was freed for withdrawal last week, but it is unclear when the North will move to get its money.
In a separate development, Macau's Banco Delta Asia said on Monday it had filed a legal challenge against the Bush administration's decision to cut it off from the US financial system.
The bank told the US treasury department's accusations "lacked specific facts" and were motivated by politics, Delta Asia said in a statement.
It said the US move was "politically motivated since it was based on disputes between the United States and North Korea."
The bank has repeatedly denied knowingly helping in North Korea's alleged illicit activities.