MUNICH, Germany (AP) ??" U.S. Defense Secretary Mark Esper on Friday defended his decision to divert billions of dollars in funding for Navy and Air Force aircraft and other military programs to help pay for President Donald Trump's promised wall on the U.S.-Mexico border.

Esper was asked by a reporter about criticism from Democrats and Republicans in Congress, including Rep. Mac Thornberry, a Republican from Texas, who called the diversion of funds contrary to the constitutional authority of Congress.

"Border security is national security," Esper said, "and national security is our mission."

He added, "The action we took is legal under the law, and so it should be no surprise, and I'll just leave it at that for now."

Esper spoke on the sidelines of an international security conference in Munich.

The Pentagon announced on Thursday that Esper approved shifting $3.8 billion in funds that Congress had previously authorized for F-35 fighter aircraft and other military programs. The money is being diverted to help the Department of Homeland Security build portions of the border wall. The Pentagon indicated that more such actions could be coming to provide additional funding for Trump's signature campaign promise.

The action drew sharp rebukes from Democrats as well as Republicans.

"Today's reprogramming request confirms once again that the President is obsessed with fulfilling a campaign promise at the expense of our national security," Rep. Adam Smith of Washington, the Democratic chairman of the House Armed Services Committee, said in a statement. "This administration has already stolen billions from the Department of Defense in order to begin building the president's vanity wall and today they are doubling down on bad policy."

Last year, despite congressional opposition, Trump faced no consequences when making similar transfers by cancelling or postponing dozens of military construction projects to free up $3.6 billion.

In reaction to Thursday's Pentagon announcement, Thornberry, the top Republican on the committee and a supporter of wall construction, said that while the administration's constitutional role is to recommend how tax dollars are spent, final choices are made by Congress.

"Once those choices have been made, the Department of Defense cannot change them in pursuit of their own priorities without the approval of Congress," he said. "Attempts to do so undermines the principle of civilian control of the military and is in violation of the separation of powers within the Constitution. The re-programming announced today is contrary to Congress' constitutional authority, and I believe that it requires Congress to take action."

Trump, while campaigning for the Republican presidential nomination in 2016, repeatedly promised that Mexico would pay for the wall, but Mexico refused.