WASHINGTON -- Andrea Thompson, who played a key role in U.S. arms negotiations with Russia, is leaving her position as the undersecretary of state for arms control.

In a brief statement on September 20, Secretary of State Mike Pompeo said Thompson's 'wealth of knowledge, experience, and leadership skills will be missed.' He did not give a reason for the departure.

Reuters cited a person familiar with the matter as saying Thompson submitted her resignation several days ago.

Thompson was confirmed in her position on April 26, 2018. She played a leading role in talks with Russia that eventually led to the collapse of the 1987 Intermediate-Range Nuclear Forces Treaty, known as the INF.

President Donald Trump announced in October 2018 that the United States would abandon the INF, citing alleged Russian violation and concerns that the bilateral treaty held Washington to restrictions while leaving nuclear-armed countries that are not signatories, such as China, free to develop and deploy the missiles.

The United States said Russia would have to scrap its Novator 9M729 missile systems and launchers or reduce their range to comply with the INF and prevent a U.S. withdrawal from the Cold War-era pact.

Thompson in December 2018 told reporters that the weapons system has a range that was not in compliance with the pact.

She said at the time that Moscow must "rid the system, rid the launcher, or change the system so it doesn't exceed the range' to bring Russia back 'to full and verifiable compliance.'

Moscow denied the missile violated INF terms and accused the United States in turn of wanting to abandon the pact so it could start a new arms race.

On February 2, Washington said it had formally notified Russia and other treaty parties of the United States' intent and suspended its obligations under the INF.

A day later, President Vladimir Putin said Russia had also suspended its obligations under the treaty.

With reporting by Reuters and AP

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