The US must “do something very different” in Afghanistan, such as placing American military advisers closer to the front lines of battle, or risk squandering all that has been invested there in recent years, the head of the Pentagon's military intelligence agency said Thursday.

The grim assessment by Marine Lt Gen Vincent Stewart, director of the Defence Intelligence Agency, comes as the Trump administration considers Pentagon recommendations to add more US and Nato troops and to deepen support for Afghan forces. The timing of a White House decision is unclear but is not expected this week.

In testimony before the Senate Intelligence Committee, Stewart said he visited Afghanistan about six weeks ago to see for himself what others have called a stalemate with the Taliban, the insurgent group that was removed from power in 2001 by invading US forces.

“Left unchecked, that stalemate will deteriorate in the favor of the belligerents,” Stewart said, referring to the Taliban. “So, we have to do something very different than what we have been doing in the past.”

He mentioned increasing the number of US and Nato advisers and possibly allowing them to advise Afghan forces who are more directly involved in the fighting. Currently, the advisers work with upper-echelon Afghan units far removed from the front lines.

If such changes are not made, Stewart said, “the situation will continue to deteriorate and we'll lose all the gains we've invested in over the last several years.” Testifying alongside Stewart, the nation's top intelligence official, Dan Coats, said the Taliban is likely to continue making battlefield gains.

“Afghanistan will almost certainly deteriorate through 2018 even with a modest increase in military assistance by the United States and its partners,” Coats said, adding, “Afghan security forces performance will probably worsen due to a combination of Taliban operations, combat casualties, desertion, poor logistics support and weak leadership.”

The Pentagon says it currently has about 8,400 troops in Afghanistan, about one-quarter of whom are special operations forces targeting extremist groups such as an Islamic State affiliate. Gen John Nicholson, the top US commander in Kabul, has said he needs about 3,000 more US and Nato troops to fill a gap in training and advising roles.

More than 2,200 US troops have died in Afghanistan since the U.S. invasion in October 2001.