US Defense Secretary Lloyd Austin cancelled a trip to Brussels for NATO and Ukraine meetings later this week as he remains hospitalized at Walter Reed medical center, according to defense official familiar with the plans.

Austin was taken to the hospital Sunday afternoon with symptoms “suggesting an emergent bladder issue,” Pentagon press secretary Maj. Gen. Pat Ryder said Sunday. He was then admitted to the hospital’s critical care unit later in the evening for additional care and close monitoring, the hospital said.

It is still unclear how long Austin will remain at the hospital, according to medical staff.

Austin was supposed to lead a meeting of the Ukraine Defense Contact Group (UDCG) in Brussels, followed by a meeting of the NATO Defense Ministerial. Austin attended the previous meeting of the UDCG on January 23rd virtually from his home.

On Sunday afternoon, Austin transferred his duties to Deputy Defense Secretary Kathleen Hicks, notifying the President, the Chairman of the Joint Chiefs and Congress, according to the Pentagon.

Austin has faced significant criticism for his failure to notify senior administration officials, including President Joe Biden, of his previous hospitalization and his diagnosis with prostate cancer. Austin is scheduled to testify before the House Armed Services Committee at the end of the month about the failure to notify officials.