Sen. John McCain on Monday blasted the Trump administration for lacking a strategy in Afghanistan where there are 8,400 U.S. troops stationed.

"Defense Secretary Mattis testified before the Senate Armed Services Committee last week that we are not winning in Afghanistan. And yet, six months into the new administration, it still has not delivered a strategy," the Arizona Republican said in a statement.

McCain, chairman of the Senate Armed Services Committee, said that if the administration fails to develop a strategy for success, "Congress will need to play a greater role."

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"After nearly 16 years of war, we are at a stalemate in Afghanistan. Worse, we have no strategy to end that stalemate and achieve victory. The recent deaths of three brave American soldiers underscore the urgency for a new strategy to turn the situation around in Afghanistan," McCain added.

He was referring to three U.S. soldiers who were killed by an Afghan soldier last week in eastern Nangarhar province. A second insider attack unfolded on Sunday in which an Afghan soldier opening fire on U.S. troops, wounding at least seven.

In response to McCain, Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford said at the National Press Club Monday that the administration will have a strategic review complete on Afghanistan sometimes in mid-July. Dunford also indicated that lawmakers should pass a new authorization for the use of military force (AUMF), but Congress has been deadlocked for years over how much authority it should offer.

The Pentagon, meanwhile, has been making plans to send additional U.S. troops to Afghanistan. While the number is currently expected to be between 3,000 and 5,000 troops, the number is still being worked out, CBS News' national security correspondent David Martin reported on Friday. The decision by Defense Secretary Jim Mattis could be announced in the next several days or couple of weeks and it follows Mr. Trump's move to give Mattis the authority to set troop levels.

During the Obama administration, McCain blamed Obama for the rise of the Islamic State in Iraq and Syria (ISIS), as a result of his decision to withdraw U.S. troops from Iraq, and he warned Obama against taking similar action in Afghanistan.

CBS News' John Bat contributed to this report.