The source said US military officials believe the movement could indicate preparation for a test launch of a missile comparable to the Hwasong-14 inter-continental ballistic missile (ICBM) or Hwasong-12 intermediate-range ballistic missile (IRBM).
Another probability could be the testing of the new Hwasong-13 ICBM (solid engine) that has a longer maximum range than the Hwasong-14.
Initially it was expected that North Korea would carry out a provocative test last Tuesday, to mark the anniversary of its ruling party's foundation.
However, speculation is now rising that the deployment of the US carrier strike group and nuclear-powered submarine to the Korean Peninsula may provide a fresh trigger for action.
"The North may carry out a simultaneous launch of ICBM and IRBM within a few days, in protest against the US's show of military might," a source told the Dong-A Ilbo.
The USS Ronald Reagan will conduct the 10-day joint drills in waters east and west of South Korea.
Starting tomorrow, the exercise will check the allies "communications interoperability and partnership," the US Navy's 7th fleet said in a statement.
As many as 40 navy vessels, including the Aegis destroyer, and attack helicopters will be deployed.
Meanwhile, the USS Michigan, an 18,000-ton submarine, which arrived in the South Korean port of Busan on Friday, is also expected to join the exercise.
Although Washington and Seoul insist that regular joint drills are defensive in nature, North Korea considers them to be rehearsals for invasion and has lashed out with weapons tests in the past.
The deployment comes at a time of heightened tension between the US and North Korea, with both President Trump and Kim Jong-un trading regular insults.
"The US military action hardens our determination that the US should be tamed with fire and lets us take our hand closer to the trigger for taking the toughest countermeasure," said a North Korean foreign ministry official, reported by state-run news agency KCNA on Friday.
Earlier this week the US flew two supersonic heavy bombers over the Korean peninsula, staging the first night-time joint aviation exercises with Japan and South Korea.