"The Trump Administration is deeply concerned by reports that the Iranian regime has imprisoned thousands of Iranian citizens in the past week for engaging in peaceful protests", the White House said in a statement. "And we expect all parties to continue to fully implement this agreement".
According to two U.S. sources, Trump had not made a decision by Wednesday, while Johnson told the British parliament on Tuesday that London was urging "our friends in the White House not to throw it away".
"Any move that undermines JCPOA is unacceptable", he said in a post on his official Twitter feed.
"We greatly value the nuclear deal with Iran, we think it is a considerable diplomatic accomplishment".
"The deal is working".
Many of Trump's top aides have reportedly been urging him to extend US sanctions relief, but Trump remained reluctant on January 10.
He accused Iran of "not living up to the spirit" of the nuclear agreement even though the International Atomic Energy Agency says Tehran is in fact in compliance. European countries are banking on the deal, as many began investing in Iran after the sanctions were lifted.More news: Row over report of presence of Lalu 'aides' in jail
The decision comes as Iran's government deals with protests over economic hardships and corruption that are linked to frustration among younger Iranians who hoped to see more benefits from the lifting of sanctions.
The sanctions, which were suspended in 2016, had cut Iran's central bank out of the worldwide financial system and imposed penalties for buying Iranian oil.
Her comments came after Deputy Assistant Secretary of State for Iraq and Iran Andrew Peek - a key Trump administration appointee at Foggy Bottom - told Voice of America that USA officials are "examining actions we can take against those individuals, like sanctions and other means".
The so-called Joint Comprehensive Plan of Action (JCPOA), agreed by President Obama in 2015, was signed by China, France, Russia, United Kingdom, Germany, the European Union, and the United States - but Mr Trump has since suggested he could renege on it.
Why is Iran's missile programme controversial?
According to the Tehran Times, Behrooz Kamalvandi, the deputy chief of the Atomic Energy Organization of Iran, said Wednesday that Iran would ramp up uranium enrichment operations - the key process for developing nuclear bombs - to levels far higher than before the 2015 deal if the USA backed out.
The resolution calls upon Iran not to "undertake any activity related to ballistic missiles created to be capable of delivering nuclear weapons, including launches using such ballistic missile technology". "It is delivering on its goal, which is keeping the Iranian nuclear program in check". But they said the issue should be kept separate from the nuclear deal.