Satellite imagery shows that up to 100 people have assembled at the main administrative area of North Korea’s nuclear test site, a gathering not seen since January 2013, a month before Pyongyang conducted its third nuclear test , a US institute monitoring the country said on Wednesday. The development is the latest in a series of signs that North Korea may be preparing to conduct a sixth nuclear test and comes as US lawmakers passed a set of bipartisan measures to step up pressure on the North against its development of ballistic missiles and nuclear weapons. Imagery of the Punggye-ri site from Tuesday shows “a heightened level of activity over the past few days”, including “one vehicle and a large contingent (70-100) of people standing in formation or watching in the courtyard of the Main Administrative Area”, the US-Korea Institute at Johns Hopkins University said on its 38 North website. “Such a gathering hasn’t been seen since January 4, 2013, which was followed by a nuclear test on February 12,” it said. Citing earlier imagery of the site, the website showed other signs of a possible nuclear test such as the laying of communications cables likely to be used for initiating a blast and collecting data from an explosion, as well as the pumping of water out of a test tunnel. The institute said North Korea knows when commercial satellites are passing overhead and usually tries to avoid activity during that time. “The fact these formations can be seen suggests that Pyongyang is sending a political message that the sixth nuclear test will be conducted soon,” it said. “Alternatively, it may be engaged in a well-planned game of brinkmanship.” Meanwhile, the US House Foreign Affairs Committee passed measures calling for enhanced sanctions on North Korea and condemned its development of an intercontinental ballistic missile. The measures also urged the US State Department to relist Pyongyang as a state sponsor of terrorism. Prior to the passage, congressman Ed Royce, chairman of the committee, said he wants the US government to focus on “better enforcement of existing sanctions” because North Korea “is using increasingly sophisticated techniques to evade existing sanctions”. Royce criticised the regime of North Korean leader Kim Jong-un over its nuclear programme and human rights abuses, and called for cutting off the flow of funds into the regime. President Donald Trump’s administration has said all options, including military action, are on the table if North Korea continues to pose threats to US allies and US forces. Kim said on New Year’s Day that North Korea is ready to test launch an ICBM, a veiled threat to strike the United States with a missile carrying a nuclear warhead. Trump tweeted the next day, “It won’t happen!” Calls for putting North Korea back on the state sponsor of terrorism list have gathered steam since the fatal February 13 poison attack on Kim Jong-nam, the half-brother of Kim Jong-un, in Malaysia, in which Pyongyang’s involvement is suspected. The designation was lifted in October 2008 under the administration of president George W. Bush over progress in negotiations for North Korea’s denuclearisation.