The United States slapped additional sanctions on Iran on Monday after the Trump administration's disputed unilateral weekend declaration that all United Nations (UN) penalties eased under the 2015 nuclear deal had been restored.

The announcement came in defiance of nearly all UN members, including U.S. allies in Europe, who have rejected U.S. legal standing to impose the international sanctions. It set the stage for a showdown at the annual UN General Assembly this week and also came as President Donald Trump seeks to portray himself as a champion for Middle East stability ahead of November's presidential election.

In addition to his actions against Iran, Trump just last week witnessed the signing of agreements normalizing relations between Israel and the United Arab Emirates and Bahrain, both of which reflect increased regional concerns about Iran. 

"The United States has now restored UN sanctions on Iran," Trump said in a statement issued shortly after he signed an executive order spelling out how the U.S. will enforce the "snapback" of the sanctions. "My actions today send a clear message to the Iranian regime and those in the international community who refuse to stand up to Iran." 

Secretary-General Antonio Guterres says the UN will not support reimposing sanctions on Iran. 

The UN chief said in a letter to the council president that "there would appear to be uncertainty" on whether or not U.S. Secretary of State Mike Pompeo triggered the "snapback" mechanism in the Security Council resolution that enshrined the 2015 nuclear deal between Iran and six major powers.

U.S. Secretary of State Mike Pompeo told media that Washington had put new sanctions on Venezuelan President Nicolas Maduro under the order, accusing Iran and Venezuela of having worked "to flout the UN arms embargo" for nearly two years.

The impetus for the U.S. action is the impending expiry of a UN arms embargo on Iran and an effort to warn foreign actors that they will face U.S. sanctions if they buy or sell arms to Iran. 

Under the 2015 nuclear deal that Iran struck with six major powers – Britain, China, France, Germany, Russia and the United States – the UN conventional arms embargo is to set to expire on October 18, shortly before the November 3 U.S. presidential election.

More pressure was piled up on Iran at this weekend. Iran's Foreign Minister Mohammad Javad Zarif said it is ready for a full prisoner exchange with the United States, in a virtual address to the Council on Foreign Relations in New York on Monday, amid heightened tension between this longtime foes.

Washington has long demanded that Iran release U.S. citizens including Iranian-American father and son Baquer and Siamak Namazi, who it says are political prisoners. Tehran denies it holds people for political reasons and has accused many of the foreigners in its jails of espionage. It says Iranians detained in the U.S., mostly for breaking sanctions, are being unjustly held.