Rising casualties among local Afghan forces who are trained and supported by American troops and the continued presence of an ISIS-affiliated group are becoming growing concerns for commanders, the top U.S. general in the region told Fox News.

Gen. John Nicholson said the Taliban killed more than 5,000 Afghans last year and wounded another 14,000 -- and the casualty rate is climbing this year.

Nicholson, who in March took command of U.S. forces in Afghanistan, spoke for the first time this weekend to a small group of reporters in Kabul.

"In one year, [U.S.-backed forces] suffered roughly double the number of casualties that we suffered in 15 years," Nicholson said.

Nicholson said the numbers were somewhat expected with Afghan security forces beginning to take the lead in armed conflicts. Afghan forces have shown "tactical success" on the battlefield against the Taliban, despite the heavy losses, he said.

“We are very concerned about Afghan casualties,” Nicholson said, though he commended the Afghan forces for their resiliency. "This army did not break...there has not been any significant Taliban battlefield success this year."

Nicholson said the increased casualties show the Afghan Army's "commitment to the fight."

The news comes as President Obama announced on Wednesday that he would slow the withdrawal of American troops from Afghanistan. Instead of reducing the force down to 5,500, Obama will keep roughly 8,400 in Afghanistan when he leaves office in January. There are roughly 10,000 U.S. troops on the ground now.

Despite the reduction in troops, Nicholson said he received all the capabilities necessary to continue the two U.S. military missions in Afghanistan: train Afghan security forces and conduct counterterrorism strikes. Nicholson said some 400 troops would be available if needed "over the horizon" or outside the country. He said "reducing overhead" would also help reduce troop size, but not the effectiveness of the force that remains behind.

Meanwhile, the Islamic State in Khorasan Province, as the ISIS-affiliate in Afghanistan is known, continues to attack civilians in the east.

ISIS is "a very significant concern for us and we want to maintain pressure on them, we will maintain pressure on them," Nicholson said.

At Forward Operating Base Fenty, a remote American and Afghan Army base in eastern Afghanistan, a local Afghan Army commander told a small group of reporters that ISIS recently launched attacks that killed women and children in southern Nangarhar Province, which borders Pakistan.

ISIS has recruited former Taliban members and is well financed, said Lt. Gen. Mohammad Waziri, commander of the Afghan Army's 201st Corps.

Waziri estimated there were between 1,500 and 2,000 ISIS fighters in his area of operations in eastern Afghanistan, covering seven provinces.

He said none of the ISIS fighters he faced originated from Iraq or Syria, but instead traveled a short distance from nearby Pakistan. Waziri said many of these fighters were disenfranchised Taliban militants switching sides.

The rise of ISIS has caused fighting among the various rival groups, including the Taliban, Waziri said. ISIS wants all jihadist groups in the area to join them, but some continue to resist.

Nicholson, speaking separately to reporters in Kabul on Saturday night, said ISIS' presence in Afghanistan is a shared concern between the American and Pakistani governments.

In January, President Obama authorized the U.S. military to begin airstrikes against an ISIS-affiliated group in Afghanistan. There have been dozens of airstrikes targeting ISIS fighters since the president made the decision to "green light" operations against the group the same way it targets Al Qaeda fighters.

At the height of its influence earlier this year, ISIS controlled nine districts in Nangarhar Province, Nicholson said. That number has currently been reduced to “two or three” districts.

“We need to keep the pressure on,” Nicholson said.

Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel. You can follow him on Twitter: @LucasFoxNews