Germany on Friday fired a salvo of warnings against Washington, cautioning it against hurting European cohesion, abandoning core Western 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.

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 American policies have indeed been scrapped.

In a bid to calm jittery partners, the White House has dispatched top generals to Europe this week.

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 short, brief statements in which he stuck close to conventional foreign policy, including on North Korea and Russia.

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 to take 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 cautioned the US against taking transatlantic ties for granted, pointing out that its 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," she said.

While pursuing a reliable relationship with Russia as a whole, allied nations must not be "going above partners' heads in bilateral negotiations," she added.

Von der Leyen reminded Washington of core values that all NATO members have signed up to.

"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 move to ban migrants from majority-Muslim nations.

Although the initial decree to block people from seven mainly Muslim nations from entering the United States was scrapped as it fell foul of the law, Trump has said he is considering drafting a new order.

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

"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 he spoke with disdain about the alliance while campaigning for election, calling it "obsolete."

He has advocated waterboarding,repeatedly praised 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 Assad had to go as part of a political solution to end the fighting.

But Trump has called for closer cooperation with Moscow in the fight against the Islamic State jihadist group in Syria, downplaying the issue of Assad's future.

With Russia's sway in the conflict growing since it launched a military intervention in support of Assad's regime, it has seized the initiative by brokering the fragile six-week-old truce on the ground and, this week, hosting separate peace talks in Kazakhstan along with Turkey.

Yet in a sign that the Trump administration will keep to existing policy, US allies said they won assurances from Tillerson that Washington backed a UN-brokered political solution to the Syria conflict, rather than the Kazakhstan talks.

A new round of United Nations-led talks is set to take place in Geneva on February 23 involving Syrian regime and rebel representatives.