As U.S. forces withdraw from Afghanistan, the battlefield they leave behind is changing dramatically and becoming more deadly.

No longer pinned down by U.S. air cover, Taliban fighters are attacking Afghan military posts in larger numbers with the aim of taking and holding ground — a shift from the hit-and-run strikes with posses of gunmen, explosives and suicide bombers.

Struggling to hold the insurgents back, the riposte from commanders of the Afghan security forces has been clear: across the country, they are now telling their men to kill captured Taliban fighters instead of taking prisoners.

The police chief of the militant-infested southern province of Kandahar, Gen. Abdul Raziq, bluntly announced the new approach last month, telling reporters: “I have ordered my men to kill these terrorists once and for all, and not take any prisoners, because they will be released anyway.”

Raziq later retracted his comments, saying they were misinterpreted, but he was nevertheless summoned to the presidential palace to explain himself.

Since then, however, other commanders have followed suit.

In the northern province of Kunduz, for example, police officials say they have no choice but to kill their adversaries because this summer’s onslaught has been so intense and captured Taliban are invariably released again to fight another day.

“Since they can’t be tried or jailed because the judicial system is corrupt, we have no other option but to eliminate them,” Kunduz police chief Mustafa Mohseni said.

Earlier this year, President Hamid Karzai ordered the release of thousands held in the main prison at Bagram, including almost 100 the United States had classed as “dangerous.”

Karzai had long complained that foreign forces locked up Afghans on dubious grounds, with no proper judicial process. The move further strained relations that were already troubled by Karzai’s refusal to sign a bilateral security deal allowing U.S. troops to stay in Afghanistan beyond 2014, when most foreign forces are scheduled to pull out.

Officials say many of those set free were battle-hardened men who simply recruited more fighters and returned to the fray.

“These Taliban released by the president’s decree will never change, they pick up weapons and fight us again,” said Abdul Khaliq, administrator of Hesarak district in the eastern province of Nangarhar.

In Baghlan province, further north, police chief Gen. Aminullah Amarkhil, has also given ‘take-no-prisoner’ orders.

“These are terrorists and criminals so it is better to finish them off on the battlefield,” he told reporters. “They have expanded their control in the province but there aren’t enough policemen to fight them.”

In this year’s summer offensive, the Taliban appears to have focused on gaining ground in strategic parts of the country, like border crossings or highways that facilitate the export of opium, the financial lifeblood of their insurgency.

The Taliban did not respond to requests for comment in this story.

The insurgents’ success has been limited: they have yet to capture an entire province, and the government says strategic assets remain broadly under its control.

Nevertheless, the mounting intensity of the Taliban’s assaults — which often involve hundreds of fighters at a time — poses an increasingly serious challenge to security forces that have long relied on NATO support from the air.

Kunduz police chief Mohseni said that the pace of attacks had doubled since the fighting season started in May, around the time opium crops have been planted and paths cleared in snowy mountains and, for the first time in years, insurgents were engaging his men in hand-to-hand combat.

“Some of the areas in the province have recently become very insecure and they (the Taliban) carry out attacks in big numbers to bring remote districts under their control,” Mohseni said.

He said the Taliban’s morale had clearly been boosted by a government-decreed ban on using heavy artillery in residential areas to avoid civilian casualties, and in particular by the lack of lack of NATO air support on the battlefield.

In Helmand, in the south-west, security forces struggled without ISAF air support last month to fend off an attack that involved about 800 Taliban. The battle lasted eight days and nearly 1,000 people, including many civilians, were killed.

Whole districts in Helmand are now essentially under Taliban control and the special forces commander there says he wants a truce to stop the loss of life as skirmishing continues.

In Nangarhar, by contrast, a similar Taliban attack was averted by ISAF airstrikes that inflicted heavy casualties and their assault was over in a morning with minimal losses inflicted on Afghan security forces.

“Air power by the foreign troops is the key component to this battle and we have lost many men simply because we couldn’t ask our foreign partners for airstrikes,” said a senior Afghan general who asked not to be named.

Afghan officials say uncertainty in Kabul over the outcome of this year’s election to choose Karzai’s successor has added to the vulnerability of the security forces.

Two months have passed since the run-off round of the election was held, but a winner has yet to emerge due to accusations of mass fraud and rivals Abdullah Abdullah and Ashraf Ghani have both claimed victory.

Karzai’s spokesman, Aimal Faizi, concedes the Taliban, flush with foreign reinforcements, are now fighting more as an organized battalion as the battlefield environment has changed. “The election has put a negative affect on the overall security situation and their aim to attack in hundreds is to bring territories under their control.”