North Korea's leader Kim Jong-un has praised the "magnificent" strides made in the country's nuclear weapons programme in a speech marking the opening of the first party congress in nearly 40 years.
Hailing the historic test of what North Korea claims was a hydrogen bomb in January, the state's leader said it had shown the world that it would not be cowed by sanctions or outside pressure.
The test, which was followed a month later by a successful long-range rocket launch, "clearly demonstrated to the whole world our undefeatable spirit and endless power ... in defiance of malicious pressure and sanctions by enemy forces," he said.
Most experts have questioned the North's H-bomb claim, saying that the detected yield from the January test was far too low for a full-fledged thermonuclear device.
Kim, 33, who was not even born when the last Workers' Party Congress was held in 1980, said in a speech on Friday that the party conclave would prove to be a "new milestone" that would lay out the future direction "of our revolutionary march".
Kim's speech was delivered to thousands of party delegates who had gathered in Pyongyang for the once-in-a-generation conclave.
Around 130 foreign journalists were invited to cover the event, but were not allowed inside the congress venue, restricted instead to watching from a spot 200 metres away.
There has been widespread speculation about the North preparing another nuclear test to coincide with the congress as a defiant gesture.
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It may also enshrine as formal party doctrine Kim's "Byungjin" policy of pursuing nuclear weapons in tandem with economic development.
Since Kim took power after the death of his father in December 2011, North Korea has carried out two nuclear tests and two successful space rocket launches that were widely seen as disguised ballistic missile tests.
Even as the international community responded with tougher sanctions, he pressed ahead with a drive for a credible nuclear deterrent with additional missile and technical tests.