WASHINGTON (Reuters) - U.S. ambassadors from around the world are flying to Washington next week for a State Department “chiefs of mission” conference despite the partial U.S. government shutdown, U.S. officials said on Friday.

Secretary of State Mike Pompeo decided to go ahead with the Jan. 16-17 meeting “because it is essential to the conduct of foreign affairs essential to national security,” a State Department spokesman said on condition of anonymity.

“The timing of this conference is crucial to the safety, security, and prosperity of the United States,” the spokesman added, saying the conference would “review the immediate global threats and challenges that affect all American citizens.”

Current and former U.S. officials who spoke on condition of anonymity were skeptical that the conference, which was last held in 2016, is vital to national security but said the agency had to make that internal determination so as to hold it during the shutdown.

The partial shutdown, which entered its 21st day on Friday, has left U.S. government departments ranging from Justice, Agriculture and Treasury to Commerce, Homeland Security and State without money to operate programs and pay their workers.

The shutdown is now tied for the longest such closure to date, and many of the 800,000 employees who have been furloughed or are working without pay missed their first paychecks on Friday.

The State Department spokesman said the agency was working to limit conference costs by using department facilities and staff and inviting speakers chiefly from within the government, adding travel “was arranged and funded” before the shutdown.