Iran's Supreme Leader has accused Washington of seeking to "devour" Iran and seize its oil

President Donald Trump has said the world would soon "find out" whether Iran's Supreme Leader Ayatollah Ali Khamenei was correct in warning that a US attack would ignite a regional war, adding that there is still time to make a deal.

In recent weeks, the United States has increased its military presence in the Middle East, deploying the aircraft carrier USS Abraham Lincoln, additional guided-missile destroyers and advanced air defense systems in what Trump has described as a "massive and beautiful armada."

Khamenei warned on Sunday that any military action against Iran would have far-reaching consequences across the Middle East, much bigger than after the US-Israeli attack last summer.

"They should know that if they start a war this time, it will be a regional war," Khamenei said in a speech in Tehran marking the anniversary of the 1979 Islamic Revolution. He accused Washington of seeking to "devour" Iran and seize its oil and natural gas resources.

Asked by reporters about the warning, Trump dismissed the remarks but left the door open to both diplomacy and force.

"Of course he is going to say that," Trump said. "Hopefully we'll make a deal. If we don't make a deal, then we'll find out whether or not he was right."

Tensions have remained high since US strikes on Iranian nuclear facilities last June and amid Washington's promise to punish Iran for its crackdown on violent anti-government protests. Trump has repeatedly criticized Iranian leaders over the unrest and has suggested that Iran needs "new leadership," while also urging protesters to continue demonstrating and "take over" state institutions.

Khamenei, in his speech, characterized the protests as a foreign-backed plot, calling the unrest a "sedition" similar to past movements against the Islamic Republic. Iranian authorities have blamed the violence on "terrorists" supported by the US and Israel in order to justify a military intervention.

The head of Iran's Supreme National Security Council, Ali Larijani, who held talks with Russian President Vladimir Putin at the Kremlin on Friday, said that progress was being made toward negotiations. Iranian Foreign Minister Abbas Araghchi also indicated that diplomacy could resume, although no formal talks with Washington were currently planned.