By Phil Stewart

KABUL (Reuters) - President Barack Obama's decision to withdraw roughly 1,400 U.S. troops from Afghanistan won't adversely impact America's mission there, a top general said before arriving in Kabul on Friday, adding that some jobs could be done from abroad.

Obama on Wednesday shelved his plans to cut the U.S. force nearly in half by year's end, from 9,800 to 5,500. He opted instead for a more limited withdrawal to roughly 8,400 troops, noting that Afghan forces still needed U.S. support battling the Taliban insurgency after nearly 15 years of war.

In his first comments since Obama's announcement, General Joseph Votel, the head of the U.S. military's Central Command, played down any impact of the looming troop withdrawal even as he acknowledged Afghan forces were suffering heavy casualties.

A recent Pentagon report to Congress said casualties among Afghan forces rose 27 percent from Jan. 1 to Nov. 15 last year.

"It's difficult. They're taking a lot of casualties. That's a concern. We'll have to pay attention to that," Votel told a small group of reporters traveling with him to Afghanistan.

"But I don't think the reductions that we are taking are going to impact the principal missions that we are doing, particularly with respect to the Afghan security forces."

Taliban forces now hold more territory in Afghanistan than at any time since the 2001 U.S.-led invasion, according to recent U.N. estimates. The Islamic State group has also established a small presence in Afghanistan.

Republican critics of Obama questioned why, given Afghanistan's security concerns, he was still intent on cutting the force at all. Senate Armed Services Committee Chairman John McCain said it was difficult to discern "any strategic rationale" for the cut.

Votel noted that Obama's decision to keep 8,400 troops in the country was welcomed by allies, who are expected to broadly reaffirm their commitments to Afghanistan at a NATO summit on Warsaw on Friday and Saturday.

"It sends a very hopeful message here for the coalition and I think it will encourage our partners to continue to contribute," Votel said.

Votel suggested the drawdown might take place gradually over coming months, with some support for the U.S. forces in Afghanistan being drawn from outside the country.

"We're going to achieve those reductions by moving some capabilities out that can be conducted over the horizon - that don’t necessarily need to be done in Afghanistan - to support the force," he said, without elaborating.

(Reporting by Phil Stewart; Editing by Robert Birsel)