North Korea has carried out three such tests in the past, most recently in February 2017 - an incident that triggered months of severe tension on the Korean peninsula.

It also carried out tests in 2006 and 2009. All resulted in the imposition of sanctions by the UN, which bars Pyongyang from nuclear tests under resolution 1718.

A report from 38 North, the website of the US-Korea Institute at Johns Hopkins University, also confirmed increased activity at the site "probably related to preparations for a detonation", based on satellite imagery.

The document highlighted "increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels".

While Pyongyang has tested devices, it is not yet believed to have mastered the process of making a nuclear warhead small enough to deliver via a missile.

【2017届高考英语二轮复习阅读理解素材：30】相关文章：

★ 【人教版】2014届高考英语一轮复习指导考点针对练：必修3 unit2《Healthy eating》

★ 2017届高考英语二轮复习课时训练：2（含解析）

★ 2017届高考英语二轮复习课时训练：16（含解析）

★ 2017届高考英语二轮复习课时训练：17（含解析）

★ 2017届高考英语二轮复习课时训练：12（含解析）

★ 2017届高考英语二轮复习课时训练：8（含解析）

★ 【优化方案】2017届高考英语二轮复习全国卷Ⅱ题型重组训练：第3组（含解析）

★ 2017届高考英语二轮复习课时训练：7（含解析）

★ 2017届高考英语二轮复习课时训练：9（含解析）

★ 2017届高考英语二轮复习课时训练：1（含解析）