The head of the UN nuclear watchdog said Monday he is "seriously" worried about the North Korean nuclear programme, which has not been subject to international inspections since 2009.

"I remain seriously concerned about the nuclear programme of the Democratic People's Republic of Korea (DPRK)," said Yukiya Amano, director-general of the International Atomic Energy Agency.

"It is nearly six years since agency inspectors were asked to leave the DPRK," he noted in remarks to the IAEA board of governors in Vienna.

"I call upon the DPRK to comply fully with its obligations under relevant Security Council resolutions," Amano said, speaking the day after Pyongyang fired two short-range ballistic missiles into the sea and vowed "merciless" retaliation over the latest round of US-South Korean joint military drills.

Pyongyang is believed to have a stockpile of up to 16 nuclear weapons fashioned from either plutonium or weapons-grade uranium.

The arms programme has prompted a raft of US and UN sanctions against North Korea, which has carried out nuclear tests in 2006, 2009 and 2013.

It also regularly launches missile tests, triggering international condemnation.

US researchers warned last week that the North Korean regime appears bent on expanding its nuclear programme, and in a worst-case scenario could possess 100 atomic weapons by 2020.