The Trump administration has officially withdrawn from the Open Skies treaty, six months after starting the process to leave.

"On May 22, 2020, the United States exercised its right pursuant to paragraph 2 of Article XV of the Treaty on Open Skies by providing notice to the Treaty Depositaries and to all States Parties of its decision to withdraw from the Treaty, effective six months from the notification date," State Department deputy spokesman Cale Brown said in a statement.

"Six months having elapsed, the U.S. withdrawal took effect on November 22, 2020, and the United States is no longer a State Party to the Treaty on Open Skies," Brown added.

The post-Cold War agreement was struck to allow nations to conduct flyovers of other allies in an attempt to collect military data and other intelligence on neighboring foreign enemies. 

In a statement issued on Sunday, Sen. Bob Menendez (D-N.J.) called the administration's withdrawal "reckless" and encouraged President-elect Joe Biden's administration to rejoin the pact once he is inaugurated.

"I strongly believe that President Trump’s decision to withdraw from the Treaty is a violation of domestic law," the ranking member on the Senate Foreign Relations Committee said. "In the 2020 National Defense Authorization Act, Congress reaffirmed its support for the Open Skies Treaty and specifically mandated the administration justify a withdrawal four months before any formal notification of withdrawal took place. President Trump brazenly ignored the law and is unilaterally imposing a politically-charged withdrawal, even after losing a presidential election.”

Trump first announced in May he would withdraw from the treaty, with Secretary of State Mike Pompeo formally submitting a notice of intent to withdraw from the pact a day later.

"While the United States, along with our Allies and partners that are States Parties to the treaty, have lived up to our commitments and obligations under the treaty, Russia has flagrantly and continuously violated the treaty in various ways for years," Pompeo said at the time. "This is not a story exclusive to just the treaty on Open Skies, unfortunately, for Russia has been a serial violator of many of its arms control obligations and commitments."

In June, Democrats on the Senate Foreign Relations Committee questioned the legality of the Trump administration's desire to withdraw from the pact. 

“The timing of your decision — less than five months before an election — is also suspect. Beginning the U.S. withdrawal from the Open Skies Treaty, without complying with U.S. domestic law or constitutional practice, is an obvious political maneuver in an attempt to bind a future administration,” senators wrote in a letter to Pompeo and former Secretary of Defense Mark Esper. “As such, we demand that you immediately discontinue your efforts to initiate the withdrawal process until Congress is provided with the requisite notification under the [National Defense Authorization Act], and the Senate has had an opportunity to weigh in on the withdrawal.”

During his first term in office, Trump and his allies have boasted that they have been tougher on Russia in recent years than any previous administration, despite claims of an inappropriately friendly relationship between the president and his Russian counterpart. 

In August, the Trump administration pulled out of the Intermediate-range Nuclear Forces Treaty with Russia, an agreement banning nuclear and conventional ground-launched ballistic and cruise missiles with ranges between 500 and 5,500 kilometers that was signed in 1987.

At the time, the administration accused Russia of violating the terms of the treaty after saying U.S. officials had "tried everything possible since May 2013” to stop Russia from building up its nuclear capability.

Rebecca Kheel contributed to this report, which was updated at 12:08 p.m.