By Arshad Mohammed and John Irish
WASHINGTON/PARIS (Reuters) - In the week since Washington offered to talk with Tehran about reviving the 2015 nuclear deal, Iran has curbed U.N. monitoring, threatened to boost uranium enrichment and its suspected proxies have twice rocketed Iraqi bases with U.S. soldiers.
In return, the United States and three allies, Britain, France and Germany, have responded with a studied calm.
The response - or lack of one - reflects a desire not to disrupt the diplomatic overture in hopes Iran will return to the table and, if not, that the pressure of U.S. sanctions will keep taking its toll, U.S. and European officials said.
Iran has repeatedly demanded the United States first ease the U.S. sanctions imposed after former President Donald Trump abandoned the deal in 2018. It would then wind down its own violations of the pact, which began a year after Trump's withdrawal.
"However much they believe the U.S. should lift sanctions first, that's not going to happen," said a U.S. official, who spoke on condition of anonymity.
If Iran wants the United States to resume compliance with the deal "the best way and the only way is to get to the table where those things will be discussed," the official added.
Two European diplomats said they did not expect the United States, or Britain, France and Germany - informally known as the E3 - to do more to pressure Iran for now despite what one described as "provocations."
One of the diplomats said the current policy was to condemn but avoid doing anything that could close the diplomatic window.
"We have to tread carefully," said the diplomat. "We have to see whether the E3 can juggle Iran's headlong rush and the U.S. hesitance to see whether we even have a path forward."
The "headlong rush" was a reference to Iran's accelerating violations of the agreement.
In the last week, Iran has reduced cooperation with the International Atomic Energy Agency, including by ending snap inspections of undeclared suspected nuclear sites.
A report by U.N. nuclear watchdog also said Iran has begun enriching uranium to 20%, above the 2015 deal's 3.67% limit, and Iran's supreme leader said Tehran could go to 60% if it wished, bringing it closer to the 90% purity needed for an atomic bomb.
The crux of the deal was that Iran would limit its uranium enrichment program to make it harder to amass the fissile material for a nuclear weapon - an ambition it has long denied - in return for relief from U.S. and other economic sanctions.
While the United States says it is still investigating rockets fired at Iraqi bases last week that house U.S. personnel, they are suspected of having been carried out by Iranian proxy forces in a long-standing pattern of such attacks.
In a demonstration of the restrained U.S. stance, State Department spokesman Ned Price said on Monday that Washington was "outraged" by the attacks but would not "lash out" and would respond at a time and place of its choosing.
The second European diplomat said U.S. leverage was still in place because President Joe Biden had not lifted sanctions.
"Iran has positive signals from the Americans. It now needs to seize this opportunity," this diplomat said.
On Wednesday, spokesman Price told reporters the United States would not wait forever.
"Our patience is not unlimited," Price said.
(Reporting By Arshad Mohammed and John Irish; Additional reporting by Simon Lewis and Humeyra Pamuk; editing by Grant McCool)