President Donald Trump said efforts by China to rein in North Korea have "not worked," signaling he may take a more aggressive approach to counter the hermit kingdom and its nuclear ambitions.

Mr Trump made the announcement as he also appeared to blame his predecessor Barack Obama over the death of Otto Warmbier, 22, the US student who died after being imprisoned for 17 months in North Korea.

The tour agency that brought Mr Warmbier to North Korea, operated largely by British ex-patriates in China, said it would stop taking Americans into the reclusive state.

Young Pioneer Tours had previously described North Korea as "probably one of the safest places on Earth to visit". However, the agency said it been "struggling to process" what happened to Mr Warmbier and the "risk for Americans visiting North Korea has become too high".

Mr Trump had previously praised the efforts of Chinese president Xi Jinping to put pressure on North Korea after the two men met at the president's Mar-a-Lago retreat in Florida in April.

But after the death of Mr Warmbier he said: "While I greatly appreciate the efforts of President Xi and China to help with North Korea, it has not worked out. At least I know China tried!"