The Obama administration has reportedly used a “secret channel” to directly warn Iran’s supreme leader, Ayatollah Ali Khamenei, against closing the Strait of Hormuz. According to the New York Times, the U.S. has communicated to Khamenei closure of the straight would be a “red line” that would elicit a U.S. response. Iran has threatened to block the Strait in response to harsher international sanctions or in the event of a military attack from the U.S. or Israel. The U.S. warning to Iran comes just days after an Iranian nuclear scientist was assassinated, the latest in a string of similar attacks. On Thursday, Defense Secretary Leon Panetta denied U.S. involvement.

Defense Secretary Leon Panetta: “We were not involved in any way, in any way, with regards to the assassination that took place there. I’m not sure who was involved. We have some ideas as to who might be involved, but we don’t know exactly who was involved. But I can tell you one thing: the United States was not involved in that kind of effort. That’s not—that’s not what the United States does.”

Amid the heightened tensions, Iran has signaled its willingness to take part in renewed international talks on its nuclear activities. On Thursday, Iran’s parliamentary speaker, Ali Larijani, said he thinks the impasse could be resolved through “serious talks” and proposed Turkey as a host state.