France, April 4 -- As tensions rise within the NATO alliance, US Secretary of State Marco Rubio was in Brussels to reassure uneasy allies, while France has voiced sharp concerns over President Donald Trump's committment to Europe's security.

France has sounded the alarm over what it sees as growing cracks in NATO unity, following US President Donald Trump's latest moves and remarks.

French Foreign Minister Jean-Noel Barrot warned the alliance is "being tested by the decisions taken and announced yesterday by President Trump," as concerns mount in Europe over Washington's long-term commitment to transatlantic security.

On Thursday, US Secretary of State Marco Rubio and newly appointed US Ambassador to NATO Matt Whitaker arrived in Bruss...