TALLINN, Estonia — President Barack Obama proclaimed an unwavering and permanent U.S. commitment to the security of its NATO allies, as he mounted a show of solidarity today with European nations anxious about Russia’s aggression in Ukraine.

During a visit to Estonia, Obama said the U.S. would send more Air Force units and aircraft to the Baltics.

Standing shoulder to shoulder with Estonian President Toomas Hendrik Ilves, Obama called Estonia’s Amari Air Base an ideal location to base those additional forces. He ticked through a list of U.S. military resources already at work in the region, and said the U.S. has a duty under the NATO charter to the alliance’s collective defense.

“It is unbreakable, it is unwavering, it is eternal. And Estonia will never stand alone,” Obama said in Tallinn, Estonia’s port capital.

Obama’s firm words came as NATO nations were preparing to commit to a more robust rapid-response force for the region, in response to the crisis between Ukraine and Russia. But shortly after Obama arrived in Europe, the office of Ukraine’s president said he and Russian President Vladimir Putin had reached agreement on a cease-fire — an unexpected development that added further uncertainty to Obama’s meetings with regional leaders.