US President Donald Trump on Monday announced a 25 percent tariff on any country trading with Iran, ramping up pressure on Tehran over its handling of a wave of protests.

"Effective immediately, any Country doing business with the Islamic Republic of Iran will pay a Tariff of 25% on any and all business being done with the United States of America. This Order is final and conclusive," Trump said on Truth Social.

Iran's main trading partners are China, Turkey, the United Arab Emirates and Iraq, according to the economic database Trading Economics.

The tariffs announcement comes as Trump mulls possible military action against Iran over lethal violence against protesters.

Trump said on Sunday the US may meet Iranian officials and he was in contact with Iran's opposition.

While airstrikes were one of many alternatives open to Trump, "diplomacy is always the first option for the president," White House press secretary Karoline Leavitt told reporters on Monday.

"What you're hearing publicly from the Iranian regime is quite different from the messages the administration is receiving privately, and I think the president has an interest in exploring those messages," she said.

Iranian Foreign Minister Abbas Araqchi has said Tehran was studying ideas proposed by Washington, though these were "incompatible" with US threats.

"Communications between (US special envoy Steve) Witkoff and me continued before ‍and after the protests and are still ongoing," he told Al Jazeera. (Agencies)