US Defense Secretary James Mattis has assured European counterparts about Washington's commitment to NATO, officials said Friday, as President Donald Trump plans to speak to the leaders of France, Germany and Russia.

Mattis spoke by phone to the defense ministers of France, Germany and Israel on Thursday, Pentagon spokesman Navy Captain Jeff Davis said.

Speaking to German Defense Minister Ursula von der Leyen, he "assured the minister of the United States' enduring commitment to the NATO alliance," Davis said in a statement.

"He thanked Minister von der Leyen for her country's leadership in NATO activities on the Eastern Flank and in Afghanistan, and acknowledged the role that Germany plays in fighting terrorism, specifically in the counter-ISIL coalition," he added, using an acronym for the Islamic state jihadist group.

"He also cited the strategic importance of Germany as the host to 35,000 US personnel, the largest US force presence in Europe," Davis said.

Mattis also stressed NATO's importance to his French counterpart Jean-Yves Le Drian, with whom he discussed "the long-time allies' security cooperation, which is stronger than ever as both countries engage side-by-side in the fight against terrorism," he added.

As the European Union's leading members, both countries have been rattled by Trump's denigration of NATO, which he has called "obsolete," and his praise of Russian President Vladimir Putin, whom they accuse of seeking to undermine Western unity.

Trump, who has often praised Putin, is expected to speak to him, German Chancellor Angela Merkel and French President Francois Hollande on Saturday, the White House said.

Trump took office last week with US relations with Russia at new Cold War-level lows amid accusations by American intelligence agencies that the Kremlin leaked hacked Democratic Party emails as part of a campaign to influence November's election.

Trump -- who has raised the prospect of easing sanctions imposed against Russia after its annexation of Ukraine's Crimean Peninsula in 2014 -- has cast doubt on whether Russia meddled in the election.

Mattis also spoke by phone with Israeli Defense Minister Avigdor Lieberman on Thursday "to underscore his unwavering commitment to Israel's security," Davis said.