(CNN)The Trump administration is expected to pull out of the "Open Skies" treaty -- which allows unarmed surveillance flights over American, Russian and several other countries' territories -- in a move that congressional Democrats warned would be a "gift" to Russian President Vladimir Putin.
It is believed the White House will make the announcement soon, a US official told CNN.
Here's a look at what's at involved.
What is the "Open Skies" treaty?
The treaty allows 34 countries to conduct unarmed surveillance flights over one another's territories -- including the US and Russia. It was signed in 1992 and went into effect in 2002.
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According to the State Department, the treaty "is designed to enhance mutual understanding and confidence by giving all participants, regardless of size, a direct role in gathering information through aerial imaging on military forces and activities of concern to them."
Why does the US want to withdraw?
The White House has so far not confirmed the expected withdrawal or reasons for it. However, a State Department spokesman's comment did hint at dissatisfaction with the treaty.
"We continue to implement the treaty and are in full compliance with our obligations under this Treaty, unlike Russia," the spokesperson said.
The US has in the past accused Moscow of imposing restrictions on flights near its exclave of Kaliningrad, an area between Poland and Lithuania where the Russian military maintains a robust presence.
Who is part of the Open Skies treaty?
The 34 member states are: Belarus, Belgium, Bosnia-Herzegovina, Bulgaria, Canada, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Georgia, Germany, Greece, Hungary, Iceland, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Norway, Poland, Portugal, Romania, Russia, the Slovak Republic, Slovenia, Spain, Sweden, Turkey, Ukraine, the UK, and the US.
Kyrgyzstan has signed, but not ratified the treaty.
What do Democrats say?
Democrats have been quick to criticize the impending announcement.
"Pulling out of the Open Skies Treaty, an important multilateral arms control agreement, would be yet another gift from the Trump Administration to (Russian President Vladimir) Putin," said the top Democrats on the House Foreign Affairs, House Armed Services, Senate Armed Services and Senate Foreign Relations committees, in a letter to US Secretary of Defense Mark Esper and Secretary of State Mike Pompeo.
"Not only is there no case for withdrawal on the grounds of national security, there has been no consultation with the Congress or with our allies about this consequential decision. Any action by this administration to withdraw from critical international treaties without the approval of the Senate is deeply concerning," wrote Sen. Bob Menendez of New Jersey, Sen. Jack Reed of Rhode Island, Rep. Eliot Engel of New York and Rep. Adam Smith of Washington state.
What does Russia say?
The Russian Embassy in Washington tweeted on Tuesday expressing support for the treaty, saying: "We consider the #OpenSkiesTreaty to be an important instrument in ensuring European security on the same level as the 2011 Vienna document on confidence- and security-building measures."
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