US national security agencies now believe proxies sympathetic to or working for Iran may have attacked four tankers off the United Arab Emirates rather than Iranian forces themselves, a U.S. official familiar with the latest US assessments said on Tuesday.The official said possible perpetrators might include Houthi rebels in Yemen and Iran-backed Shi'ite militias based in Iraq but said Washington did not have hard evidence on who sabotaged the four vessels, including two Saudi tankers, off Fujairah emirate on Sunday.