STATE DEPARTMENT /PENTAGON - VOA's Carla Babb contributed to this report.
The U.S. Defense Department has released new photographs it says are more proof that Iran attacked two foreign oil tankers in the Gulf of Oman last week.
The pictures show what the United States says are Iranian Revolutionary Guard forces taking an unexploded mine off the side of the Japanese-owned Kokuka Courageous tanker. Another shot shows where the mine had been attached.
Other pictures show a large hole in the Courageous that the Pentagon says was likely caused by another mine.
Press photographs taken after Thursday's attacks showed a Norwegian tanker on fire, sending thick black smoke into the sky.
"Iran is responsible for the attack based on video evidence and the resources and proficiency needed to quickly remove the unexploded limpet mine," a Pentagon statement accompanying the photos said.
Iran has not reacted to the new pictures, but has denied involvement in the tanker attacks, calling the claims "ridiculous" and "dangerous."
Monday afternoon, Acting U.S. Defense Secretary Patrick Shanahan announced the deployment of about 1,000 more troops to the Middle East for what he said were"defensive purposes."
"The recent Iranian attacks validate the reliable, credible intelligence we have received on hostile behavior by Iranian forces and their proxy groups that threaten United States personnel and interests across the region," Shanahan said in a statement.
House Speaker Nancy Pelosi said Iran needs to be held accountable for its "dangerous activities in the region," but that U.S. actions must be "strong, smart and strategic -- not reckless and rash."
"This deeply concerning decision may escalate the situation with Iran and risk serious miscalculations on either side," she said in a statement.
Iranian Foreign Minister Mohammad Javad Zarif has said the timing of the tanker attacks is "suspicious," because a Japanese tanker was hit while Prime Minister Shinzo Abe was in Tehran seeking to defuse U.S.-Iran tensions.
Earlier Monday, the U.S. called Iran's plan to surpass an internationally agreed limit on its stock of low-enriched uranium "nuclear blackmail."
"President [Donald] Trump has made it clear that he will never allow Iran to develop nuclear weapons. The regime's nuclear blackmail must be met with increased international pressure," White House National Security Council spokesperson Garrett Marquis said.
Top State Department officials urge the international community not to yield to Iran's "nuclear extortion."
"We continue to call on the Iranian regime not to obtain a nuclear weapon, to abide by their commitments to the international community," spokeswoman Morgan Ortagus told reporters. She called Iran's announcement "unfortunate" but not surprising.
The U.S. government's comments followed Tehran's announcement that it would soon surpass the limit on the amount of enriched uranium it is allowed to keep under the 2015 nuclear deal with six world powers.
French President Emmanuel Macron said he regrets the Iranian announcement, urging Tehran "to behave in a way that is patient and responsible." Britain said if Iran exceeds the nuclear limits it would consider "all options."
Britain and France signed the deal with Iran, along with China, Germany, Russia, and the United States.
Iran's nuclear agency spokesman Behrouz Kamalvandi said on state television Monday, "The countdown to pass the 300 kilograms reserve of enriched uranium has started and in 10 days time [June 27] ... we will pass this limit."
But he said Iran would be open to going back to observing the limit if it gets help from other signatories to the agreement in circumventing U.S. sanctions on its vital oil industry.
Tensions between the U.S. and Iran are escalating more than a year after Trump pulled the U.S. out of the nuclear agreement and reimposed sanctions on Iran.
President Hassan Rouhani announced last month Iran would stop observing restrictions on its stocks of enriched uranium and heavy water agreed to under the 2015 nuclear deal. He pointed out that the U.S. has dropped out of the pact, reimposed old sanctions, and added new ones.
Iran is also angry that the other parties to the nuclear agreement have not done enough to help the battered Iranian economy recover from the sanctions while still insisting Iran keep its part of the bargain.
Trump had called the 2015 agreement "horrible" and said he would like to negotiate a new one. But the United Nations atomic watchdog agency says Iran has continued to meet terms of the 2015 pact. While Washington has pulled out of the deal, the other signatories have not.