The US is sending an additional 1,000 troops to the Middle East in response to “hostile behavior by Iranian forces and their proxy groups”, Patrick Shanahan, the acting defence secretary, announced on Monday.

The move further heightens tensions between the two countries. Last week Washington blamed Iran for attacks on two oil tanker ships, which came more than a year after Donald Trump announced that the US was withdrawing from a 2015 nuclear deal and restoring economic sanctions.

“I have authorized approximately 1,000 additional troops for defensive purposes to address air, naval, and ground-based threats in the Middle East,” Shanahan said in a statement.

“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.”

The acting secretary added: “The United States does not seek conflict with Iran. The action today is being taken to ensure the safety and welfare of our military personnel working throughout the region and to protect our national interests.”

Officials told the Associated Press the deployment included security forces and troops for additional surveillance and intelligence gathering and was among options initially laid out to US leaders late last month. Those options included as many as 10,000 troops, along with Patriot missile batteries, aircraft and ships.

As relations worsen, Iran also said on Monday that it would soon breach limits on how much enriched uranium it can stockpile under the nuclear deal, officially called the Joint Comprehensive Plan of Action (JCPOA).

The accord, which was brokered by Barack Obama and which Iran and the other signatories have maintained following Trump’s withdrawal, caps Iran’s stock of low-enriched uranium at 300kg enriched to 3.67%. But Iran said it had quadrupled the rate of enrichment, so that in 10 days it would bypass the 300kg limit.

A spokesman for the White House National Security Council said this amounted to “nuclear blackmail”.

Morgan Ortagus, a spokeswoman for the state department, told reporters on Monday: “We continue to call on the Iranian regime not to obtain a nuclear weapon, to abide by their commitments to the international community.

“It’s unfortunate that they have made this announcement today. It doesn’t surprise anybody and this is why the president has often said that the JCPOA needs to be replaced with a better deal.”

The US decision to pull out, and Trump’s “America first” approach to foreign policy, appears to have damaged trust with allies, some of whom are seeking stronger proof that Iran was behind the oil tanker attacks.

Mike Pompeo, the secretary of state, told CBS’s Face the Nation on Sunday: “Yes, we’ve shared it with allies already. You’ve had the chance to see it. I made a bunch of phone calls yesterday, I’ll make a whole bunch more calls today. The world needs to unite against this threat from the Islamic Republic of Iran.”

On Monday, the Pentagon released new images it said showed Iran was behind the attack on one of the ships. The case rests on an unexploded limpet mine on the Kokuka Courageous tanker ship that the US says was removed by Iranians on a patrol boat.

“Iran is responsible for the attack based on video evidence and the resources and proficiency needed to quickly remove the unexploded limpet mine,” the Pentagon said in a statement accompanying the imagery.

Critics, meanwhile, accuse the Trump administration of manufacturing a crisis. Ben Rhodes, former national security adviser to Obama, tweeted: “Trump pulling out of the Iran Deal has led directly and predictably to the Iranian nuclear threat getting worse and the risk of war going up.”

Democrats urged caution and called for the White House to provide an explanation. Nancy Pelosi, the House speaker, said: “Americans must have no illusions about the Iranian regime, and must remain committed to holding Iran accountable for its dangerous activities in the region. But we must be strong, smart and strategic – not reckless and rash – in how to proceed. The Congress must be immediately briefed on the administration’s decisions and plans.

“This deeply concerning decision may escalate the situation with Iran and risk serious miscalculations on either side. Diplomacy is needed to defuse tensions, therefore America must continue to consult with our allies so that we do not make the region less safe.”

On Tuesday China urged restraint, saying the US risked “opening a Pandora’s Box” in the Middle East.

State councillor Wang Yi said, “the US side should alter its extreme pressure methods. Any unilateral behaviour has no basis in international law”, and warned that it could create “an even greater crisis”. Wang also urged Iran not to abandon the nuclear deal.

Agencies contributed reporting