The warning came as NATO's allies signed off on the accession protocols for Sweden and Finland.
Deputy Secretary General Mircea Geoană said he "anticipates" allied presence to remain in Eastern Europe following Ukraine invasion.
NATO's Secretary General Jens Stoltenberg has promised the accession process would be "smooth and swift."
A move by Sweden and Finland to join NATO would be a colossal foreign policy shift. Russia has warned the two countries against joining the alliance.
President Joe Biden met with world leaders in Brussels to address Russia's invasion of Ukraine. Live updates have ended.
Russian Deputy Foreign Minister Andrei Rudenko warned against plans for its neighbor to join the alliance.
Sen. Elizabeth Warren said three years of the Trump presidency has put the country's alliances under "enormous strain."
In a clip from a NATO reception that went viral on Wednesday, world leaders including Boris Johnson appeared to make fun of the U.S. president.
The reason Giuliani must have made the calls was "not so complicated," the president said Wednesday.
Queen Elizabeth II's only daughter was spotted with Justin Trudeau, Emmanuel Macron and Boris Johnson at a Buckingham Palace reception Tuesday.
The president is visiting the U.K. for the NATO summit in London from December 3-4.
Only 37 percent of Germans believe the continent needs U.S. protection.
The Republican senator called Trump's performance at NATO "disappointing yet ultimately unsurprising."
The president made the claim in an unscheduled press conference Thursday and said he told allies, "I would be very unhappy if they didn't up their commitment very substantially."
The summit is an opportunity to present an updated strategy for the transatlantic alliance, but President Trump is unlikely to follow the script.
Trump boasted that the NATO leadership is very fond of him, but says Russia must be thinking "if we did like this guy, we made a big mistake."
The letters said America's contribution to the NATO alliance is "no longer sustainable."
They're not the most obvious picks for Melania's new buddies.
After two years of NATO's defense building, should the Kremlin be worried?
The Ministry of Defense wants a new center to coordinate Baltic deployments.
Two aircraft carriers will be in Mediterranean as Washington seeks to balance Russia's military activities and accelerate its fight against ISIS.
Poland's defense minister has confirmed discussions with NATO's Secretary General.