The United Nations and the European Union on Monday warned Iran not to step up its nuclear programme and risk ending more than two years of negotiations between Tehran and the west.

All five permanent members of the United Nations Security Council – the US, Britain, France, China and Russia – have delivered diplomatic démarches to Iran’s mission in Vienna, calling on Tehran to step back from its threat to resume “research and development” that could involve uranium enrichment – a process that can lead to weapons grade material – in pilot projects.