Iran's missile capabilities are its red line and are not a subject to be negotiated, an adviser to Iran's supreme leader said on Wednesday, as Tehran and Washington eye a new round of talks to avert conflict.

U.S. and Iranian diplomats held indirect talks last week in Oman, amid a regional naval buildup by the U.S. threatening Iran.

"The Islamic Republic's missile capabilities are non-negotiable," Ali Shamkhani said according to state media while appearing in a march commemorating the 47th anniversary of the Islamic Revolution.

Washington has long sought to extend talks on Iran's nuclear capabilities to cover its missile program as well. Iran has said it is prepared to discuss curbs on its nuclear program in return for the lifting of sanctions, but has repeatedly ruled out linking the issue to other questions including missiles.

Israel's Prime Minister Benjamin Netanyahu is expected to use a meeting with Trump in Washington on Wednesday to push for any U.S.-Iran deal to include limitations on Tehran's missiles.

On Sunday, Iran's Foreign Minister Abbas Araqchi said Tehran's missile program had never been part of the talks' agenda.