TEHRAN - The United Nations has expressed concern about the increasingly confrontational rhetoric between the United States and Iran and called for calm.

Amid rising tensions between Washington and Tehran, U.S. President Donald Trump tweeted early on May 20, "If Iran wants to fight, that will be the official end of Iran."

Iranian Foreign Minister Mohammad Javad Zarif dismissed Trump's "genocidal taunts" and warned the U.S. president not to threaten Iran.

"We are concerned about the rising rhetoric," said UN spokesman Stephane Dujarric, according to AP.

"We would ask all parties to lower the rhetoric and lower the threshold of action as well," Dujarric said.

Dujarric's remarks came amid concerns about a potential military conflict between the United States and Iran.

Washington has ordered a beefing up of U.S. military assets in the Middle East and Persian Gulf, claiming "imminent threats" from Iran and ordered the evacuation of personnel from the U.S. embassy in neighboring Iraq

Tehran has dismissed the U.S. allegations, and accused Washington of an unacceptable escalation of tensions.

Both sides have said they do not want a war.

In a separate tweet on May 20, Trump said if Tehran wants to negotiate, it will have to take the first step, AFP reported.

"Iran will call us if and when they are ever ready. In the meantime, their economy continues to collapse -- very sad for the Iranian people!" Trump tweeted.

Iranian President Hassan Rouhani was quoted by local media as saying on May 20 that he favored talks and diplomacy but not under current conditions.

"Today's situation is not suitable for talks and our choice is resistance only," Iran's IRNA news agency quoted Rouhani as saying.

Later on May 20, Trump told reporters: "With Iran, we'll see what happens, but they've been very hostile.”

"I think Iran would be making a very big mistake if they did anything. If they do something, it will be met with great force but we have no indication that they will," Trump said.

"We'll have no choice," he added.

Relations between Iran and the United States plummeted a year ago when Trump pulled out of the 2015 nuclear deal which curbed Iran's nuclear program in exchange for relief from crippling sanctions.

Since then, Washington has stepped up its rhetoric and reimposed sanctions, while the Western European parties to the accord said they remained committed to it, but failing to make sure of Iran’s interests in the deal.

In announcing the U.S. pullout from the nuclear agreement between Iran and world powers, Trump claimed the terms were not tough and did not address Iran's missile program or Tehran's rising power in the region.

Iran denies it supports insurgent activity, including in Yemen, and has said its nuclear program is strictly for civilian energy purposes.

Earlier this month, Iran said it was suspending several commitments under the nuclear pact, and threatened to step up uranium enrichment if European countries did not act to protect it from the effects of the U.S. sanctions.

SP/PA