UN secretary-general Ban Ki-moon has said North Korea has cancelled an invitation for him to visit a factory park in the country that is the last major co-operation project between the rival Koreas.

Mr Ban had previously said he wanted to go on Thursday to the Kaesong industrial park just north of the heavily fortified Korean border to help improve ties between North and South Korea, which jointly run the complex but have seen their always-tense ties worsen in recent weeks.

The former South Korean foreign minister would have been the first UN chief to visit the factory park, which opened in 2004 and is a rare legitimate source of foreign currency for the impoverished North, and the first UN head to visit North Korea since Boutros Boutros-Ghali in 1993.

North Korea gave no reason when it told the UN of its decision to cancel his trip, Mr Ban told a forum in Seoul.

Analysts had said Mr Ban's trip was not likely to bring any major breakthrough in ties between the Koreas, and some have calculated North Korea made a last-minute decision to cancel because it was unlikely to get much out of such a visit.

"This decision by Pyongyang is deeply regrettable," Mr Ban said, adding that he will spare no effort to encourage North Korea to work with the international community for peace and stability on the Korean Peninsula and beyond.

Relations between the Koreas are strained over North Korean missile and other weapon tests that South Korea views as provocations. There are also worries after South Korea's spy agency said last week North Korean leader Kim Jong Un had his defence chief executed by anti-aircraft gunfire in late April.

North Korea appears to have determined that Mr Ban would only back the views of Seoul and Washington during his trip, said Lim Eul Chul, a North Korea expert at South Korea's Kyungnam University.