SEOUL (Reuters) - North Korea has tightened security ahead of a ruling party congress, South Korea said on Friday, with authorities keen to avoid any "mishap" at the gathering at which advances in the drive for nuclear weapons will likely be hailed.

Thousands of delegates are expected in the capital, Pyongyang, from May 6 for the first congress in 36 years at which young leader Kim Jong Un is expected to cement his leadership and formally declare the country a nuclear-armed state.

"Strengthening security can be seen as a measure to prevent mishaps over the party congress," Cheong Joon-hee, spokesman at South Korea's Unification Ministry, which oversees dealings with the North, told a briefing.

North Korea has in the past taken such steps ahead of major events and has at times also shut down its border with China for the same reason, Cheong said.

North Korea announced the Workers' Party congress in October but only confirmed the May 6 starting date on Wednesday.

The Daily NK, a website run by defectors with sources in North Korea, said that since mid-April, free movement in and out of the capital has been stopped and security personnel have been summoned from the provinces to step-up domestic surveillance.

The congress, expected to last four or five days, will be closely watched for any new policies and for how North Korea presents its pursuit of nuclear weapons, which has intensified since January when it conducted its fourth nuclear test.

The nuclear test was followed with a string of missile tests, though not all successful. On Thursday, it tested what appeared to be two intermediate-range ballistic missiles but both failed, the U.S. and South Korean militaries said.

South Korea, and others nervously watching the North's defiance of U.N. resolutions aimed at curbing its nuclear and ballistic missile technologies, expect another nuclear test before the congress.

North Korean authorities have also enlisted people in Pyongyang and some other places in a 70-day campaign to ramp-up productivity and spruce-up the capital, the Daily NK reported.

(Reporting by Jack Kim; Editing by Robert Birsel)