* Tillerson to visit Brussels, Vienna and Paris
* Trump denies reports of plan to replace Tillerson with Pompeo
* Europe troubled by Trump decisions over Iran, climate change
By Robin Emmott and Noah Barkin
BRUSSELS/BERLIN, Dec 3 (Reuters) - On the eve of his trip to Europe, Rex Tillerson gave a speech last week that European allies had waited months to hear: an "ironclad" promise of U.S. support to its oldest allies.
The relief in European capitals lasted barely a day as reports surfaced of a White House plan to oust the U.S. secretary of state, plunging America's friends back into confusion over President Donald Trump's foreign policy.
The uncertainty is particularly acute given Washington's leading role in crises in North Korea and Syria.
"Just as Tillerson comes to Brussels to give a public statement of support that the EU and NATO have wanted all along, it seems he has no mandate, that the guillotine is hanging over his head," said an EU official involved in diplomacy with White House officials.
"It leaves Europe just as doubtful as before about Trump."
U.S. officials said on Thursday the White House had a plan for CIA Director Mike Pompeo to replace Tillerson but Trump said on Friday he was not leaving and the secretary of state said on Saturday the reports were untrue.
European leaders yearn for stability in U.S. foreign policy. They are troubled by Trump's "America first" rhetoric and inconsistent statements on NATO and the European Union.
In addition, Trump's decision to pull out of the Paris climate change accord and his decision not to certify Iran's compliance with a nuclear deal undermine European priorities.
"The chaos in the administration doesn't help in the current geopolitical climate," said a senior French diplomat.
Early last week, Tillerson, a former Exxon Mobil chief executive, delivered a long address in support of Europe in Washington more akin to traditional U.S. policy.
"The United States remains committed to our enduring relationship with Europe. Our security commitments to European allies are ironclad," he told a think tank.
He said he would convey that message to the European Union and NATO. He is set to visit Brussels on Tuesday and Wednesday, the Organization for Security and Cooperation in Europe in Vienna on Thursday and Paris on Friday.
The question is whether European officials believe him, given tensions during his April visit to Europe, when Reuters reported Tillerson initially planned to skip a NATO meeting in Brussels and then only attended under pressure from allies.
"If there were expectations that Tillerson might evolve into a counterweight to Trump, someone who could pass on messages from partners and exert moderating influence over American foreign policy – those expectations have been disappointed," said Niels Annen, foreign policy spokesman for Germany's Social Democrats in parliament.
"On his watch, the State Department has become a non-actor."
Despite Tillerson's pledge to reform the U.S. foreign service, European governments take a dim view of how he has sought to cut costs at the State Department, with top diplomatic posts unfilled almost a year into the administration.
The French have gone around Tillerson to develop contacts with U.S. Secretary of Defense Jim Mattis, White House National Security Adviser H.R. McMaster and White House Chief of Staff John Kelly, while the EU's top diplomat Federica Mogherini has gone directly to Vice President Mike Pence.
Berlin has focused on Capitol Hill, as well as Kelly, McMaster and Mattis.
Yet it is unclear if that access translates into a direct impact on Trump's foreign policy, diplomats said.
"LARGER DOSE OF TRUMP"
There is hope that if Pompeo is appointed he could rejuvenate the State Department after Tillerson, who is seen as ineffective, diplomats said. Pompeo is an unknown quantity in Europe but is viewed as closer to Trump.
"We may be looking at a larger dose of Trump at the State Department," if Pompeo did get the job, said Thomas Kleine-Brockhoff, head of the German Marshall Fund's Berlin office.
One European diplomat said Tillerson was in a difficult position from the outset because the Trump administration was hostile to Iran and brought in a team of generals who took a hard line, "so it never left Tillerson much room."
In addition, Trump's son-in-law Jared Kushner has taken a leading role in formulating policy on Middle East peace.
But Europeans see Trump as a blizzard of conflicting signals. At a NATO summit in Brussels in May, the president publicly admonished European leaders for their low defence spending and threatened to reduce support, only to announce a jump in U.S. military spending in Europe months later.
Things may only become more unpredictable, diplomats say.
European diplomats see Tillerson and Mattis as instrumental in talking Trump out of making any rash decisions over North Korea and its nuclear program, given administration comments about "utterly destroying" the country. (Additional reporting by John Irish in Paris, Gabriela Baczynska in Brussels, Linda Sieg and Noburiro Kubo in Tokyo; Editing by Matthew Mpoke Bigg)
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