US President-elect Donald Trump has laid into White House press secretary Josh Earnest, describing him as "a foolish guy" in an ongoing battle between Trump and Obama's aides.

Trump's comment came as spokespeople for President Barack Obama and the president-elect have increasingly butted heads in a series of antagonistic statements.

"Having the right press secretary is so important," Trump said during a rally in Hershey, Pa, The Hill reported.

"This foolish guy, Josh Earnest. He delivers their message so bad. He could deliver a positive message bad."

"The president's very positive, but he's not so positive. Maybe he's getting his orders from someone else. Does that make sense?"

Earnest had recently issued a statement on the news that Russian president Vladimir Putin was believed by US intelligence agencies to be personally involved in the hack on the US elections, claiming Donald Trump knew Russia was responsible.

Earnest's statement read: "There's ample evidence that was known long before the election and in most cases long before October about the Trump campaign and Russia — everything from the Republican nominee himself calling on Russia to hack his opponent.

"It might be an indication that he was obviously aware and concluded, based on whatever facts or sources he had available to him, that Russia was involved and their involvement was having a negative impact on his opponent's campaign."

But Trump's Transition Team senior advisor Kellyanne Conway slammed the message, stating on Fox News: "This is incredibly disappointing to hear from the podium of the White House press secretary because he essentially stated that the president-elect had knowledge of this, maybe even fanned the flames.

"I guess he's auditioning to be a political pundit after his job is over soon. And I wonder if his boss, President Obama, agrees."

However, on Friday (16 December), President Obama attempted to downplay the in-fighting between the teams, telling The Hill: "I think they would be the first to acknowledge we've done everything we can to ensure that they're successful.

"It's been a few days since I talked to [Trump] about the transition. ... That cooperation is going to continue."