Defense Secretary James Mattis' new authority to manage troop levels in Afghanistan will likely mean the deployment of thousands of additional U.S. service members this summer.

Testifying on Capitol Hill this week, Mattis said his authorization to manage troop levels in that Middle Eastern country "does not at this time change the troop numbers for Afghanistan." But Mattis is expected to approve the deployment of 3,000 additional troops to increase the number of trainers and advisers, as well as the number of special operations forces.

"We are not winning in Afghanistan right now," Mattis said in his congressional testimony Tuesday.

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Exactly when the new troop levels will go into effect is unclear, but the Trump administration strategy for Afghanistan is expected to be complete by mid-July, as Mattis testified. President Trump still has not been presented with an Afghanistan strategy. That's still in the works -- even though troop levels and an overall strategy were supposed to be finalized before the NATO meeting in Brussels, Belgium, last month.

The White House indicates it can send more trainers now that an Obama-era cap was lifted. The justification to lift the cap on levels ahead of the formation of an actual strategy is that Afghan President Ashraf Ghani's government was shaken by recent bombings in the country, and additional U.S. support will help stabilize the situation.

Roughly 8,400 U.S. troops are already serving in Afghanistan.

Three U.S. soldiers were killed and another wounded Saturday when they were attacked by an Afghan soldier, who was killed. Also, two U.S. Army Rangers died in an April 27 raid on an Islamic State compound.

The U.S. has been waging war in Afghanistan since October 2001, when U.S. and British forces began airstrikes in Afghanistan after the Taliban refused to hand over now-dead al Qaeda leader Osama Bin Laden, blamed for the Sept. 11, 2001, terrorist attacks. Although the U.S.-led coalition ended its combat mission against the Taliban in 2014, U.S. forces are still helping Afghan forces on the battlefield.

CBS News' David Martin and Margaret Brennan contributed to this report.