U.S. Vice President Mike Pence said on Monday that his country will continue to pile up economic pressure on Iran and defend U.S. interests as Tehran went one step further to endanger the 2015 nuclear deal.

"Let me be clear: Iran should not confuse American restraint with a lack of American resolve," Pence told audience at an annual summit of a pro-Israel Christian organization in Washington.

"We hope for the best, but the United States of America and our military are prepared to protect our interests and protect our personnel and our citizens in the region," he added.

Pence's remarks came hours after Tehran announced that Iran has raised the concentration of its enriched uranium to 4.5 percent, crossing the limit of 3.67 percent set by the landmark nuclear pact.

In his speech, Pence also vowed that his country will continue to bring pressure on Iran's economy, which he claimed had already been "reeling" under heavy U.S. sanctions.

Pence's tough stance was echoed by U.S. Secretary of State Mike Pompeo and National Security Adviser John Bolton, both Iran hawks, who spoke later on the same event.

"We've implemented the strongest pressure campaign in history against the Iranian regime, and we are not done," Pompeo noted.

"President Trump's maximum pressure campaign against Iran is working," Bolton told the same group. "We're just getting started. [...] The president's goal is to get a new deal that would be negotiated in the best interests of the United States."

U.S. President Donald Trump, for the second time, spoke over phone with his French counterpart Emmanuel Macron on Monday over Iran.

Trump and Macron discussed "ongoing efforts to ensure that Iran does not obtain a nuclear weapon" and Iran's "destabilizing behavior" in the Middle East, according to a statement from the White House.

Farhan Haq, deputy spokesman of UN Secretary-General Antonio Guterres, said Guterres is aware of the announcement that Iran may have begun enriching uranium beyond the 3.67-percent limit set by the Joint Comprehensive Plan of Action, using the formal name of the deal.

"If confirmed, such action by the Islamic Republic of Iran would neither help preserve the plan nor secure tangible economic benefits for the Iranian people," Guterres said.

A year after the U.S. unilateral exit from the nuclear deal, Iran withdrew from part of the nuclear deal on May 8 and threatened more actions in case its interests under the pact cannot be guaranteed.

Iran said earlier that the Europeans have failed to take "practical" steps to ensure Iran's economic interests under the deal.

Around a week ago, Iran announced that its low-grade enriched uranium stockpile had exceeded 300 kg, its first breach of limits set in the accord.

UN Secretary-General Antonio Guterres believes Iran's action to cross the nuclear deal limit would neither help preserve the plan nor secure economic benefits for the Iranian people, his deputy spokesman said Monday.

(Cover: U.S. Vice President Mike Pencethe speaks to the annual Christians United for Israel conference in Washington, DC, U.S., July 8, 2019. /Reuters Photo)

(With input from agencies)