WASHINGTON (Reuters) - A top U.S. military commander said on Tuesday he has recommended keeping 13,600 American troops in Afghanistan after 2014, when the NATO combat mission ends and most Western troops withdraw.

U.S. Marine Corps General James Mattis testifies before the Senate Armed Services Committee in Washington March 5, 2013, with regard to the Defense Authorization Request for fiscal year 2014. REUTERS/Gary Cameron

General James Mattis told the Senate Armed Services Committee he believed that properly designing the residual force would be “critical” for a good outcome to America’s longest war.

“I have made my recommendation,” Mattis said in response to a question from Republican Senator John McCain. “That recommendation is for 13,600 U.S. forces.”

The figure provided by Mattis, who leads the U.S. military’s Central Command, is above the range of troop levels U.S. officials have said were being considered by the White House and discussed by NATO defense chiefs last month.

The White House said Obama had not yet made a decision.

“The president is continuing to review a range of options for a possible U.S. troop presence after 2014. We’re not going to discuss numbers at this stage,” said White House spokeswoman Caitlin Hayden.

The decision is a delicate one for Obama, who wants to put an end to the war launched after the September 11, 2001, attacks but also ensure that a large enough American force stays behind to provide training support for Afghan forces and launch counterterrorism operations against al Qaeda and its allies.

U.S. officials have said in the past that the White House was considering up to 9,000 troops, a lower range than put forward initially last year by the U.S. and NATO commander in Afghanistan, General John Allen.

Allen, who stepped down last month, initially suggested keeping between 6,000 and 15,000 troops in Afghanistan.

Asked how many troops NATO allies might contribute, Mattis cautioned that “I assume it’d probably be around 50 percent of what we provide.”

NATO defense chiefs meeting in Brussels on February 22 discussed keeping a combined NATO force of between 8,000 and 12,000 troops, which would include American forces, the Pentagon said at the time. That compares to combined NATO force of about 100,000 troops in Afghanistan now.

Should Obama opt to keep less than 13,600 U.S. troops in Afghanistan after 2014, Mattis’ recommendation could give ammunition to Republicans in Congress who have accused the Democratic president of failing to heed the advice of military commanders.

In his State of the Union address last month, Obama announced that 34,000 troops, or about half the American forces in Afghanistan now, would withdraw by early next year.

Mattis said he supported that decision because the troops would be able to stay through peak summer fighting months.

Mattis also added his voice to calls for keeping the Afghan forces at their peak strength of 352,000 longer, as opposed to current plans to cut them by roughly a third after 2015. NATO officials are strongly considering such a proposal.

General Lloyd Austin, nominated to succeed Mattis as the next head of Central Command, has also backed keeping the Afghan forces at their peak strength.