Iran has warned the United States that it is ready for war if Washington wants to "test" its resolve, as US President Donald Trump recently escalated pressure, saying that any country doing business with Iran will face 25 percent tariffs from the US.

Oil prices climbed on Tuesday in direct response to market jitters over the Middle East situation, as Iran accounts for about 3 percent of global oil production.

Trump announced his decision on Monday via social media, saying the tariffs would take effect "immediately".

Meanwhile, the virtual US embassy in Iran urged US citizens to leave the country immediately by land and reach Armenia or Turkiye. With no physical US embassy or consulate operating in Iran, the Swiss embassy acts as the protector of US interests there.

China has denounced the US' tariff move. Addressing a regular news briefing on Tuesday in Beijing, Foreign Ministry spokeswoman Mao Ning said: "China's position on the tariff issue is very clear. Tariff wars have no winners. China will resolutely safeguard its legitimate rights and interests."

Mao said that China is closely monitoring the situation in Iran, and will take all necessary measures to protect the safety of Chinese citizens.

"China hopes Iran will maintain its stability, and supports Iran in doing so. China has always opposed interference in other countries' internal affairs and the use or threat of use of force in international relations. It hopes that all parties will act in ways conducive to peace and stability in the Middle East," she added.

Trump's new move piles additional pressure on Iran, coming close on the heels of the threat on Sunday that his administration is weighing "some very strong options", including potential military action, against the Middle Eastern nation.

In an interview with Al Jazeera on Monday, Iranian Foreign Minister Seyed Abbas Araghchi said that channels of communication with the US are open, but emphasized that his country was "prepared for all scenarios", claiming that Iran now has "large and extensive military preparedness" compared with last year's 12-day war with Israel.

The New York Times reported on Monday that the Pentagon is presenting Trump with a "wider range of strike options" on Iran than previously reported. In June last year, the US joined Israel's war on Iran by conducting airstrikes on Iranian nuclear facilities.

According to the report, which quoted a US official, possible targets include Iran's nuclear program and ballistic missile sites. However, the official also said that covert cyber operations or a strike against Iran's domestic security apparatus are more likely.

"If Washington wants to test the military option it has tested before, we are ready for it," said Araghchi, adding that he hopes the US will choose "the wise option" of dialogue.

Karoline Leavitt, White House press secretary, said on Monday that "airstrikes are among the numerous options on the table for the commander-in-chief", but diplomacy has always been President Trump's "first option".

Violent protests erupted across Iran in late December following the sharp depreciation of the national currency and soaring living expenses. The death toll of civilians and security personnel has reportedly crossed 600 and is still mounting.

State media reported that tens of thousands of pro-government demonstrators took to the streets of Tehran and other cities on Monday, waving the national flag and condemning the US and Israel for "instigating unrest" in the country.

"These massive rallies have thwarted the plan of foreign enemies that were supposed to be carried out by domestic mercenaries," Iranian Supreme Leader Ayatollah Ali Khamenei was quoted as saying by state television.

During the Al Jazeera interview, Iranian Foreign Minister Araghchi said his communications with US Special Envoy Steve Witkoff over nuclear talks had "continued before and after the protests, and are ongoing".

However, he questioned whether the US was "ready for fair and just negotiations", as Washington's proposed ideas and threats against Iran are "incompatible" with peaceful dialogue.