North Korea says its main nuclear facility, the Yongbyon complex, has resumed normal operations.

The country was improving its nuclear weapons "in quality and quantity", state-run news agency KCNA reported.

Yongbyon's reactor was shut down in 2007 but Pyongyang vowed to restart it in 2013, following its third nuclear test and amid high regional tensions.

The reactor has been the source of plutonium for North Korea's nuclear weapons programme.

Experts believe that if re-started, the reactor could make one bomb's worth of plutonium per year.

The announcement about Yongbyon is the first official confirmation from North Korea that it has restarted operations there.

A US think-tank said earlier this year that satellite images suggested that work had started at the plant.

KCNA said on Tuesday that the North was ready to face US hostility with "nuclear weapons any time".

However, the full scope of North Korea's nuclear capabilities is unclear.

Pyongyang claims it has made a device small enough to fit a nuclear warhead on to a missile, which it could launch at its enemies.

However, US officials have cast doubt on this claim and experts say it is difficult to assess the progress North Korea has made on miniaturisation.

BBC