US National Security Adviser John Bolton has said "naval mines almost certainly from Iran" were to blame for the damage to oil tankers in the Gulf of Oman earlier this month. Mr Bolton provided no evidence to support the allegation, which Iran said was "laughable". The attacks off the eastern coast of the United Arab Emirates on 12 May left holes in the hulls of four ships. The incident came amid an escalation in tensions between Iran and the US. Last Friday, US Vice Admiral Michael Gilday said he believed "with a high degree of confidence that this [attack] stems back to the leadership of Iran at the highest levels". Mr Bolton, a long-standing advocate for regime change in Iran, echoed the admiral's words during a visit to the UAE on Wednesday, telling reporters it was "clear that Iran is behind" the attack. "There's no doubt in anybody's mind in Washington who's responsible for this," he said. "Who else would you think is doing it? Someone from Nepal?" But Abbas Mousavi, Iran's foreign ministry spokesman, said Mr Bolton's accusations were "ludicrous". "Iran's strategic patience, vigilance and defensive prowess will defuse mischievous plots made by Bolton and other warmongers." In another development, Iran's Supreme Leader Ayatollah Ali Khamenei has said Iran will not negotiate any new deals with the US over its nuclear and missile programmes, calling negotiations "useless". Earlier President Hassan Rouhani signalled talks were possible if sanctions were lifted.