President Barack Obama announced during a press conference at the White House on Monday that Defense Secretary Chuck Hagel resigned from his position.

The president said Hagel, who began serving as Defense Secretary since February 2013, will remain in the position until the Senate confirms a successor.

During his speech, Obama described Hagel as an "an exemplary Defense Secretary" and "a great friend," and praised his leadership effort at the Pentagon.

"When it's mattered most behind closed doors, in the Oval Office, you've always given it to me straight. For that, I will always be grateful," Obama said, according to NBC News.

Obama also suggested that Hagel's resignation was his own choice.

"If there's one thing I know about Chuck it is that he does not make this or any decision lightly," the president said. "This decision does not come easily to him."

However, inside sources say that the former Nebraska senator was forced out by the president, CNN reports. According to officials, the White House lost confidence in Hagel's ability to effectively lead in the Pentagon. Plus, the former Republican senator faced pressure as criticism of the president's national security team on a series of global issues mounted, including the threat of the Islamic State.

"He wasn't up to the job," said one senior official, reports NBC.

Other sources said that Hagel was expected to bring the wars in Iraq and Afghanistan to an end. However, he was not a good fit to take on the Islamic State ramped up.

"Rather than winding down two wars, we're winding up," said an insider.

However, Arizona Republican Sen. John McCain revealed that Hagel had his own frustrations with the Obama Administration.

"I know that Chuck was frustrated with aspects of the Administration's national security policy and decision-making process," McCain said in a statement. "His predecessors have spoken about the excessive micro-management they faced from the White House and how that made it more difficult to do their jobs successfully. Chuck's situation was no different."

McCain also spoke about Hagel's frustration and resignation on Arizona radio station KFYI 550's "The Mike Broomhead Show."

"They're gonna say well it was time for a change and all that ... but I can tell you he was in my office last week, he was very frustrated," McCain said adding there was a lack of strategy to combat ISIS, help the Ukranians and what McCain called "a lack of U.S. influence ... unknown in history."

McCain added that despite White House people 'leaking' that Hagel "wasn't up for the job," he disagrees.

"... believe me, he was up to the job," McCain said. "It was the job that he was given where he really was never really brought into that real tight circle inside the White House that makes all the decisions," he said.