Munich (Germany) (AFP) - European leaders Friday fired a salvo of warnings against Washington, cautioning it against hurting EU cohesion, abandoning shared values and seeking a rapprochement with Russia behind the backs of its allies.

In a hard-hitting speech at the Munich Security Conference against President Donald Trump's administration, German Defence Minister Ursula von der Leyen urged the United States not to take transatlantic ties for granted.

"Our American friends know well that your tone on Europe and NATO has a direct impact on the cohesion of our continent," the German minister told the Munich Security Conference.

"A stable European Union is also in America’s interest, as is a strong and unified NATO," she said.

Polish President Andrzej Duda also stressed that "the basis of our strength is our transatlantic bond. We must not recklessly put it into question.

"We all realise that the consequence of a potential destabilisation on Europe will be felt on both sides of the Atlantic," he added.

Trump's praise for Britain’s decision to quit the EU, his criticism of NATO, and his softer approach towards Russia have rattled allies, prompting them to seek assurances from his lieutenants on whether long-standing US policies have indeed been scrapped.

As jittery partners wonder what foreign policy under Trump would look like, the White House has dispatched top generals to Europe this week to offer reassurances.

Vice President Mike Pence is due to address the Munich conference Saturday, a day after US Defense Secretary Jim Mattis affirmed to the same forum that the bond between Europe and America is the "strongest bulwark" against instability and violence.

"I am confident that we will strengthen our partnerships, confronting those who choose to attack innocent people or our democratic processes and freedoms," he told the gathering of security and defence experts.

Separately in Bonn, where foreign ministers of G20 nations closed a two-day meeting, US Secretary of State Rex Tillerson made brief statements in which he stuck close to conventional foreign policy, including on North Korea and Russia.

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With the White House embroiled in controversy over its ties to the Kremlin, Tillerson was cautious in his dealings with Moscow, despite Trump's pledges for closer ties.

Following his first sitdown with Russian Foreign Minister Sergei Lavrov on Thursday, Tillerson said the US sought cooperation with Moscow only when doing so "will benefit the American people".

- Not equi-distance -

In Munich, Germany's defence minister urged the US to cherish transatlantic ties, pointing out that allies should not be put on the same footing as Russia.

"There cannot be a policy of equi-distance to allies and to those who openly question our values, our borders and international law," von der Leyen said, adding that allied nations must not be "going above partners' heads in bilateral negotiations."

Von der Leyen also reminded Washington of NATO's core values.

"That never leaves any room for torture. It requires us to prevent causing any civilian victims and it means giving protection to those who are in need."

She also took aim at Trump's plan to ban migrants from majority-Muslim nations.

"We should be careful that this fight (against terror) does not become a front against Islam and Muslims.

"Otherwise we run the risk of digging ourselves into a deeper grave in which violence and terror only grow further. Rather, it is right to seek partnership with like-minded Muslim and Arab nations," she said.

- 'Obsolete' -

Since taking office Trump has said he fully backs NATO, but called it "obsolete" while campaigning for election.

He has advocated waterboarding, repeatedly praised Russian President Vladimir Putin and called for better cooperation with Russia, including in the fight against the Islamic State group in Syria.

Under Trump's predecessor Barack Obama, Washington insisted President Bashar al-Assad had to go as part of a political solution in Syria -- an issue Trump has downplayed.

Ukrainian President Petro Poroshenko warned that moving in the direction of appeasement with Russia "would be naive, wrong and dangerous".

Seeking to allay allies' concerns, senior US Senator John McCain noted that Trump sometimes "contradicts himself, so some of us have learnt to watch what the president does as opposed to what he says.

"Make no mistake these are dangerous times but you should not count America out," McCain added.

British Foreign Secretary Boris Johnson had a good word for the Trump administration.

"Give these guys and give Donald Trump a chance. Yes it’s a new style of government, but don’t underestimate a new approach to deliver results," Johnson said.