North Korea's nuclear and missile capabilities are accelerating at an "unprecedented" pace, the South's acting president said Monday, urging the swift deployment of a US anti-missile system that has infuriated Beijing.

Seoul and Washington agreed last year to install the Terminal High Altitude Area Defence (THAAD) system in the South after a string of North Korean nuclear and missile tests -- prompting strong objections from China, which fears it will undermine its own ballistic capabilities.

Speaking to reporters on Monday, Acting President Hwang Kyo-Ahn said the North had conducted two nuclear tests and fired multiple ballistic missiles last year, averaging two per month.

"Its nuclear and missile capabilities are developing at an unprecedented pace," Hwang said. "They are real and obvious threat."

A "swift" deployment of the THAAD system "is necessary to protect the lives and safety of our people", he added.

Within South Korea, voices opposing the installation have grown louder, with some opposition candidates pledging to scrap the agreement if they win a presidential election due this year.

The plan has also angered Beijing, which has imposed a string of measures seen in the South as economic retaliation, including effectively barring K-pop stars from performing on the mainland and not authorising South Korean airlines to operate charter flights between the countries.

"There are various concerns about economic retaliation from THAAD but the relationship between South Korea and China did not happen in just one or two years," Hwang said.

He took over President Park Geun-Hye's sweeping executive powers after she was impeached in December, becoming the de facto leader of Asia's fourth largest economy.

Park is accused of colluding with close friend Choi Soon-Sil to strong-arm companies into handing over tens of millions of dollars to dubious foundations which Choi controlled.

The case is now being considered by the Constitutional Court -- which has up to six months to reach a ruling -- but hundreds of thousands of South Koreans have joined weekly protests calling for Park's immediate departure from office.

If the court confirms the impeachment, a presidential election will have to be held within 60 days.