If you're walking around Washington DC early Wednesday, you may encounter blaring sirens, stressed emergency service personnel, police cordons and utter pandemonium. Not to worry, it’s part of a ‘full scale’ terrorist attack drill.

Hundreds of police, fire, and emergency medical service personnel, as well as volunteer actors, will take part in the exercise, designed to prepare for the possibility of “a complex coordinated terror attack in the National Capital Region.”

The drill will be staged at six sites in the District of Columbia, suburban Maryland and Northern Virginia, according to a statement from the Metropolitan Washington Council of Governments. It occurs just hours before the entire US Senate attend a major White House briefing on North Korea.

DC FEMS will play active role today in @MWCOG regional anti-terrorism exercise. Keeping communities safe thru coordinated partnerships. — DC Fire and EMS (@dcfireems) April 26, 2017

ACFD is participating in a regional terror drill. We hope this event never occurs in our community, but must be prepared if it does @DHSgovpic.twitter.com/C5hbQb9HYT — Arlington Fire (@ACFDPIO) April 26, 2017

The drill will start at 7:30am EST. The public have been reassured that it will take place in a “controlled environment,” and that there is no cause for alarm.

Scott Boggs, managing director of Homeland Security and Public Safety at the Council of Governments said that law enforcement officials regularly practice their skills on their own but this drill will “go one step further and stage a very realistic emergency event involving multiple sites and actors posing as the casualties.”

The drill comes as the Trump administration prepares to hold a major briefing on North Korea later on Wednesday.

All 100 US senators have been called to the White House for the briefing by Secretary of State Rex Tillerson, Secretary of Defense Jim Mattis, Director of National Intelligence Dan Coats and General Joseph Dunford, chairman of the Joint Chiefs of Staff, said White House spokesman Sean Spicer on Monday.

Reuters described the meeting as unusual, noting that it was rare for the entire Senate to go to the White House, and for all four of those officials to be involved.