US President Barack Obama on Wednesday admitted the "inherent limitations" of the NATO air operation in Libya, but predicted a slow, steady, process would eventually force Muammar Gaddafi from power.

Both Obama and British Prime Minister David Cameron urged patience and persistence in the operation at a press conference in London.

"Once you rule out ground forces, then there are going to be some inherent limitations to our air strike operation," Obama said.

Obama hit out at the "false perception" that there were super-secret assets that were kept "in a warehouse" somewhere but had the potential to turn the tide in the war.

"Ultimately, this is going to be a slow, steady process," Obama said, but added that the opposition in Libya would have to "carry out its responsibilities" to oust Gaddafi.