BRUSSELS — NATO ambassadors are meeting today to weigh U.S. plans to withdraw from an international treaty allowing short-notice observation flights over more than 30 countries, amid concerns that Russia is violating the pact.

President Donald Trump said Thursday that Russian violations make it untenable to stay in the Open Skies Treaty. Washington has signaled that it will pull out in six months, although Trump hinted that he might reconsider the decision.

The treaty came into force in 2002. It was meant to promote trust between the United States and Russia by allowing signatories to conduct reconnaissance flights over each other’s territories to collect information about military forces and activities.