WASHINGTON - Story updated July 10, 2019, 3:05 p.m.

President Donald Trump said Wednesday he would soon "substantially" increase economic sanctions against Iran, even as the U.S. accused Tehran of "nuclear extortion" by breaching the 2015 international pact aimed at curtailing its nuclear weapons development.

Iran has acknowledged it is now enriching uranium beyond the limits of the accord Trump withdrew from last year and keeping a bigger stockpile than it was allowed.

On Twitter, Trump contended that "Iran has long been secretly 'enriching,' in total violation" of what he called "the terrible 150 Billion Dollar deal made by (former U.S. Secretary of State) John Kerry and the Obama Administration" in agreeing to return money the Iranian government was owed at the same time the international nuclear deal was signed four years ago.

"Remember, that deal was to expire in a short number of years. Sanctions will soon be increased, substantially!" Trump declared.

Trump's remarks came as the U.S. told an emergency International Atomic Energy Agency meeting in Vienna, "There is no credible reason for Iran to expand its nuclear program, and there is no way to read this as anything other than a crude and transparent attempt to extort payments from the international community."

Washington called on Iran "to reverse its recent nuclear steps and cease any plans for further advancements in the future."

But the U.S. said it remains open to new negotiations with Tehran without preconditions, and held out "the possibility of a full normalization of relations."

Iran is already reeling from sanctions Trump reimposed on the Islamic Republic when he pulled the U.S. out of the nuclear accord. Tehran has been appealing to the remaining signatories to the deal - Britain, France, Germany, the European Union, Russia and China - to help it overcome the crippling effect of the U.S. move to curb Iran's international financial transactions and global oil trade.

Jackie Wolcott, the U.S. ambassador to international organizations, told the United Nations atomic watchdog agency that Iran's "misbehavior" should "not be rewarded."

Tehran appears to have lost patience with the European countries in seeking relief from the U.S. sanctions.

"Negotiations are never possible under pressure," Iranian Foreign Minister Mohammad Javad Zarif said as he met with French diplomatic adviser Emmanuel Bonne in Tehran.

The five remaining signatories to the deal have called on Iran to adhere to the pact's requirements. Zarif said it was up to the European allies of the U.S. to resolve issues surrounding Trump's withdrawal from the pact.