WASHINGTON — U.S. warplanes in Afghanistan have launched airstrikes against Taliban targets in recent days under new authority granted to the top commander there to help Afghan forces achieve "catastrophic success," Defense officials said.

The expanded authorities allow Army Gen. John Nicholson to bolster Afghan offensives with U.S. combat advisers, airstrikes, surveillance aircraft and pilots to fly alongside the fledgling Afghan air force, according to a Defense official who spoke on condition of anonymity because officials were not authorized to speak publicly about the new missions.

Nicholson alone has the authority to order the new missions, the Defense official said. The four-star Army officer took command in March and has been assessing the security situation there. Expanding the U.S. combat role grew out of that assessment.

Defense Secretary Ash Carter announced the expanded U.S. role in Afghanistan last week at a meeting with NATO defense ministers in Brussels. Within days, the first missions were flown, said Col. Michael Lawhorn, a military spokesman in Kabul. Since there have been only a few airstrikes, it's too early to assess their effects, he said.

The expanding U.S. combat role comes as the Taliban insurgency has bounced back three years after the Afghan troops took the lead for providing security in their country. "Increased insurgent fighting in urban areas has also contributed to record-high civilian casualties, primarily caused by insurgent and extremist groups," according to a Pentagon report released this week.

There are about 10,000 U.S. troops in Afghanistan, 6,800 of them trainers and the remainder involved in counter-terrorism operations. President Obama has called for that force to be cut nearly in half by year's end.

The decision to unleash more U.S. firepower during the traditional peak fighting season stems from the need to help Afghan forces "generate strategic effects on the battlefield," Carter said last week in Brussels.

× Fear of missing out? Sign up for the Early Bird Brief - a daily roundup of military and defense news stories from around the globe. Thanks for signing up. By giving us your email, you are opting in to the Early Bird Brief.

An example of the type of mission that can be approved would be surveillance from drones, airstrikes and combat advisers to help Afghan troops retake a provincial capital seized by the Taliban, the Defense official said.

Prior to last week’s change, U.S. forces could attack enemies in Afghanistan for three reasons: self-defense; counter-terrorism strikes against al-Qaida terrorists, its affiliates and Islamic State fighters; and racing to the aid of Afghan forces facing catastrophic loss of key terrain or suffering significant casualties, the official said.

The new, fourth authority is envisioned to help Afghan troops achieve "catastrophic success," the official said.

Obama had hoped to end the U.S. military involvement in Afghanistan by the end of his administration, leaving behind a small troop presence at the embassy in Kabul. But the resurgent Taliban and the uneven performance of Afghan troops forced the reassessment and continued need for U.S. forces there.

NATO officials announced last week that they intended to conduct operations throughout Afghanistan at least through next year, rather than pulling back to the capital, Kabul. NATO also committed to funding Afghan security forces through 2020.