Seoul - Satellite imagery indicates a high level of activity at North Korea's nuclear test site as the North seeks to keep the facility ready for a try-out at all times, a US think tank said on Monday.The US-Korea Institute at Johns Hopkins University said satellite imagery from July 7 of the North's Punggye-ri shows what appears to be supplies and/or equipment stacked next to the site's North Portal where the North conducted its fourth nuclear test in January.A small vehicle is also present at the nearby support building and several mine ore carts may also be present south-east of the portal, suggesting the tunnel is being actively worked."Based on imagery alone, it is not possible to determine whether this activity is for maintenance, excavation or preparation for a fifth nuclear test", it said on its website 38 North."Nevertheless, it is clear that North Korea is ensuring that the facility is in a state of readiness that would allow the conduct of future nuclear tests should the order come from Pyongyang", it added.Tensions are high since Pyongyang carried out its fourth nuclear test in January, followed by a series of missile launches.Pyongyang threatened on Monday to take "physical action" after Washington and Seoul announced they would deploy a sophisticated US anti-missile defence system.The North's military, which it said was possessed of "sufficient latest offensive strike means", said in response that it would take "more merciless and powerful successive corresponding measures" against the deployment of the Terminal High Altitude Area Defence (THAAD) in the South.Pyongyang test-fired what appeared to be a submarine-launched ballistic missile on Saturday, a day after the THAAD announcement.Also on Monday, the North said it would cut off all diplomatic channels with the US and handle issues related to it under its "wartime law" in protest at sanctions on leader Kim Jong-Un.The issue of two Americans currently in jail in the North is "no exception", the North said, indicating the North would give up using the two as a negotiating card and treat them harshly.State Department spokesperson John Kirby on Monday urged Pyongyang to cease an "improper and unjust" detention of them."We call on North Korea to refrain from actions and rhetoric that only further raise tensions in the region", Kirby said at a regular press briefing.Otto Warmbier, a 21-year-old college student, was sentenced to 15 years' hard labour in March for stealing a propaganda banner from a hotel. Korean-American missionary Kim Dong-Chul is also serving a jail term on charges of subversion and espionage.The latest sanctions, which described Kim directly responsible for a long list of serious human rights abuses, are the first that name the leader of the isolated, nuclear-armed state.