By Phil Stewart
WASHINGTON, June 25 (Reuters) - Acting U.S. Defense Secretary Mark Esper will be ready to update European allies on tensions with Iran as he heads to NATO headquarters this week during his inaugural trip as Pentagon chief, a senior U.S. official said.
Esper, whose first full day in the Pentagon was on Monday, led the Army until the surprise resignation of Patrick Shanahan as acting defense secretary last week. Esper is now the third person in six months to work at the defense secretary's desk.
His first week coincides with a previously scheduled NATO defense ministerial meeting, where the focus will be on other pre-planned discussions - including an approaching end to the 1987 Intermediate-range Nuclear Forces (INF) Treaty between the United States and Russia.
But Kathryn Wheelbarger, an acting assistant defense secretary, said Pentagon officials would raise Iran during one of the NATO sessions on Wednesday or Thursday.
"We are prepared to provide an update to the (NATO) alliance," Wheelbarger said, briefing reporters ahead of the trip.
"It's very important to the (Defense) Department and the U.S. government as a whole that we make sure our allies are as cognizant, and that we are as transparent on this issue, as possible."
U.S. President Donald Trump nearly waged military strikes on Iran last week after it shot down a U.S. drone and on Monday announced new sanctions targeting Iranian Supreme Leader Ayatollah Ali Khamenei and other senior Iranian officials.
Trump, who withdrew from a 2015 nuclear deal with Iran despite objections from European allies, wants to force Tehran to open talks on its nuclear and missile programs and its activities in the Middle East.
Iran has so far refused and instead warned it is ready to enrich uranium to a higher level if Europe cannot shield Tehran from U.S. sanctions. Iran denies U.S. accusations that it has also repeatedly carried out attacks on oil tankers in recent weeks.
France, Britain and Germany have sent an official diplomatic warning to Iran about the serious consequences Tehran faces if it scales back its compliance with the 2015 nuclear deal, two European diplomats said on Monday.
For many NATO allies, this week's NATO defense ministerial will be a chance to get a sense of Esper and the direction he'll take the Pentagon.
It will be a similar opportunity for Esper, who sent guidance to U.S. forces on Monday saying the Pentagon's priorities - including goals like strengthening U.S. alliances - remained unchanged.
"A NATO ministerial is a good way to get to know key partners, kind of like diplomatic speed-dating," said Derek Chollet, a former senior Pentagon official during the Obama administration.
Chollet said allies would be closely watching for hints about the kind of role Esper will play, including whether he might be like Jim Mattis, Trump's first defense secretary who was a strong advocate for NATO and was seen as a moderating influence on the U.S. president.
Mattis, who resigned in December over policy differences with Trump, brought Esper into the job.
One European diplomat joked: "Jim Mattis is not someone we can clone, as much as we'd like to, but Esper is talked about positively."
"Everyone will want to make a good impression and to get some time because he is the new face of the Pentagon," the diplomat said. (Reporting by Phil Stewart in Washington; additional reporting by Robin Emmott in Brussels Editing by James Dalgleish)
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