CAIRO, Feb. 13 (Xinhua) -- A senior Iranian official reiterated Friday that Tehran's missile program remained a nonnegotiable "red line," as the United States ordered a second aircraft carrier to the Middle East amid stalled diplomatic efforts.

Ali Shamkhani, a senior political adviser to Iran's Supreme Leader Ali Khamenei, said in an interview with Al Jazeera that Iran would respond "decisively and appropriately" to any military action.

"Iran's missile power is among its red lines and is not subject to negotiation," he said, adding that the military remained on high alert and warning that the cost of any "miscalculation" by outside powers would be high.

Still, he said talks between Tehran and Washington could advance and safeguard mutual interests if they were rooted in realism and avoided excessive demands.

Later in the day, U.S. President Donald Trump said the USS Gerald R. Ford, the world's largest aircraft carrier, had been ordered to join the USS Abraham Lincoln and three guided-missile destroyers already deployed to the region.

Trump said he was weighing military options if negotiations failed to yield a new nuclear agreement.

"I'll talk to them as long as I like, and we'll see if we can get a deal," he told reporters on Thursday. "And if we can't, we'll have to go to phase two. Phase two will be very tough for them."

The two countries held indirect talks in Muscat, Oman, on Feb. 6, but tensions have remained high amid a U.S. military buildup near Iranian waters. Speaking Wednesday at events marking the 47th anniversary of Iran's 1979 Islamic Revolution, Iranian President Masoud Pezeshkian blamed a Western "wall of distrust" for slowing progress.

Washington has said any deal with Iran must include a ban on uranium enrichment, the removal of already enriched material, limits on long-range missiles, and a rollback of support for regional proxies. Analysts said such conditions would be "very difficult" for Iran to accept.

Despite the friction, the International Atomic Energy Agency (IAEA) has resumed dialogue with Tehran following recent strikes on Iranian targets.

At the Munich Security Conference, IAEA chief Rafael Grossi called the situation "complex and extremely difficult," but said inspectors had returned to most sites except those recently attacked.

"We are at a very, very crucial moment," he said. "We might be seeing the light at the end of the tunnel, in terms of having something over the next few days."

Tehran has not commented on Grossi's remarks. Iran had previously suspended cooperation with the agency after strikes on its nuclear facilities that it attributed to Israel and the United States.

Under the 2015 nuclear accord, formally known as the Joint Comprehensive Plan of Action, Iran agreed to cap uranium enrichment at 3.67 percent. Since the United States withdrew from the agreement in 2018, Tehran has exceeded those limits. Western officials say Iran now possesses uranium enriched to 60 percent, a level that could be further refined to weapons-grade material.

Iran denies seeking nuclear weapons and says its program is intended for peaceful energy purposes.