US allies said they had won assurances Friday from new Secretary of State Rex Tillerson that Washington backed a political solution to the Syria conflict, ahead of UN-led peace talks.

Tillerson used a G20 gathering in Germany to hold a series of meetings with his global peers, reviewing crises from North Korea to Ukraine at a time of great uncertainty over President Donald Trump's "America First" strategy.

Washington's top diplomat joined a group of countries who support the Syrian opposition for talks on a way to end the nearly six-year war.

"All the participants want a political solution because a military solution alone won't lead to peace in Syria," German Foreign Minister Sigmar Gabriel told reporters in Bonn, adding that "Tillerson became very involved in the debates".

The meeting of the so-called "like-minded" nations -- made up of around a dozen Western and Arab countries as well as Turkey -- was the first since President Donald Trump took office.

Diplomats had said before the talks that they were hoping for clarity on whether there had been a change in US policy on Syria, particularly on the future of President Bashar al-Assad.

The meeting came ahead of a new round of United Nations-led talks in Geneva on February 23 involving Syrian regime and rebel representatives.

Under Trump's predecessor Barack Obama, Washington insisted Assad had to go, putting it at odds with Moscow which has launched a military intervention in support of the Syrian leader.

But Trump has called for closer cooperation with Moscow in the fight against the Islamic State jihadist group in Syria, downplaying what happens to Assad.

- 'No parallel negotiations' -

With Russia's sway in the conflict growing, it has seized the initiative by hosting separate peace talks in Kazakhstan along with Turkey, brokering a fragile six-week-old truce on the ground.

Gabriel said the "like-minded" countries had agreed to step up pressure on Russia to back a political solution and reaffirmed there could be no alternative to the UN-led Geneva talks.

"There should not be any parallel negotiations," he said.

French Foreign Minister Jean-Marc Ayrault, speaking alongside Gabriel, said a key stumbling block was Russia's insistence to consider all opposition groups as "terrorists".

A Western diplomatic source said Tillerson had stated clearly in the meeting that "there would be no military cooperation with Russia until they distance themselves from Damascus's stance on the opposition."

The former ExxonMobil boss, who kept a low profile and steered clear of press conferences, reassured allies by sticking close to conventional US foreign policy.

In his first one-on-one with Chinese counterpart Wang Yi, Tillerson pressed Beijing to help rein in North Korea after its series of nuclear and ballistic missile tests.

Tillerson "urged China to use all available tools to moderate North Korea's destabilising behaviour," acting US State Department spokesman Mark Toner said.

The talks marked the highest level China-US encounter yet after the two powers got off to a rocky start under Trump.

The US billionaire initially angered Beijing by questioning Washington's decades-old "One China" policy.

Wang only agreed to go to Bonn after a conciliatory phone call between Trump and President Xi Jinping in which the US president backtracked and reaffirmed the position that Taiwan is not separate from China.

- 'Reassurance' -

With the White House embroiled in controversy over its ties to the Kremlin, Tillerson was cautious in his dealings with Moscow, signalling there would be no immediate rapprochement despite Trump's pledges to take a softer line despite the unresolved Ukraine crisis.

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".

After the G20 on Friday, the politics moves to the high-profile Munich Security Conference where US Vice-President Mike Pence will make his international debut.

"He's going to reassure our allies of our commitment to our European partners and the reassurance for the transatlantic alliance," a senior White House adviser said.

In a speech on Saturday, Pence is expected to confirm Washington's support for NATO while pressing member states to follow through on pledges to share more of the defence spending burden.