WASHINGTON — (AP) — Secretary of State Antony Blinken announced Monday that one of his most senior aides is leaving for a top job with Defense Secretary Lloyd Austin and will be replaced by a brother of President Joe Biden’s national security adviser.

State Department counselor Derek Chollet, a veteran of national security positions in Democratic presidential administrations, will become Austin’s chief of staff. Chollet will be replaced in his State Department role by Tom Sullivan, brother of Biden's national security adviser, Jake Sullivan. Tom Sullivan had been Blinken's deputy chief of staff for policy.

Chollet had been nominated to be undersecretary of defense for policy, but his Senate confirmation was stalled, and administration officials had become increasingly frustrated by the repeated delays.

The musical chairs in national security hierarchy comes amid increasing concern over developments in the Middle East, Ukraine and China.

“I am delighted that Derek Chollet will start in July as my chief of staff. He is one of the most distinguished, far-sighted and skillful national-security practitioners of his generation, and I am grateful to him for taking on this key assignment at such an important moment,” Austin said in a statement.

Blinken said in his own statement: “During his time at the State Department, he has taken on some of the toughest diplomatic assignments, from the Balkans and Burma to Pakistan and Northern Ireland, and for the past nine months has been key to shaping our response to the crisis in the Middle East."

Blinken also praised Tom Sullivan, who had been a senior official under former Secretary of State John Kerry, as having “an unmatched depth of knowledge about how national security policy is formulated and implemented, one that he brings to bear every day on behalf of the American people.”

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