Donald Trump on Thursday alarmed nonproliferation experts when he called for the U.S. to "greatly strengthen and expand its nuclear capability." The comments, made by the president-elect via Twitter, came the same day Russian president Vladimir Putin also vowed to strengthen his country's nuclear weapons capabilities.

"Can a tweet start an arms race? This one may just have done that," Joseph Cirincione, president of Ploughshares Fund, a global security foundation, told NBC News.

The Ploughshares Fund estimates that there are currently over 15,300 nuclear weapons in the world, over 90 percent of which are currently in possession of the two former cold war foes — the United States and Russia. According to the think tank Nuclear Threat Initiative, the two countries each keep nearly 2,000 nuclear weapons in a state of high alert, so that they are ready for immediate launch.

Over the past few years, President Barack Obama has made efforts to modernize the so-called U.S. nuclear triad — ballistic missile submarines, bombers, and land-based missiles. However, no U.S. president has called for an actual expansion of the country's nuclear arsenal in decades.

"The president-elect’s latest take to Twitter should be deeply alarming to the American public and people everywhere," Derek Johnson, executive director of Global Zero, a group that advocates elimination of all nuclear weapons, said in a statement in response to Trump's comments. "These are not ordinary military capabilities. Nuclear weapons are designed to wipe entire cities off the map and inflict catastrophic loss of life. The use of even a single nuclear weapon, anywhere in the world, would be a global humanitarian, environmental and economic disaster."

Trump spokesman Jason Miller, however, said that the tweet did not imply that the president-elect was advocating the use of nuclear weapons.

"President-elect Trump was referring to the threat of nuclear proliferation and the critical need to prevent it — particularly to and among terrorist organizations and unstable and rogue regimes," Miller said. "He has also emphasized the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength."

This is not the first time Trump has talked about the U.S.' nuclear arsenal. During the first presidential debate in September, Trump said that he couldn't "take anything off the table," even while acknowledging that "once the nuclear alternative happens, it's over."