WASHINGTON — President Trump will issue a proclamation on Wednesday directing the Department of Defense and the Department of Homeland Security to work with governors to deploy National Guard troops to the southwest border to assist the Border Patrol in combating illegal immigration.

“It’s time to act,” said Kirstjen Nielsen, the secretary of homeland security, as she outlined the policy during a White House briefing.

In recent days, and in anticipation of an annual increase in numbers of people who attempt to cross the border, the Trump administration has been ramping up plans to block migrants and asylum seekers, including young unaccompanied children, from entering the United States.

The announcement came a day after Mr. Trump said he wanted to send the military to the southwest border to guard against growing threats from unchecked immigration, suggesting he might want to use active-duty armed forces to do what immigration authorities cannot.