Speaking to a visiting U.S. congressional delegation, President Hamid Karzai said the July withdrawal date had provided "morale value" to the insurgency, the presidential office said.
Karzai also told the head of U.S. Central Command, Gen. James Mattis, that terrorism could not be defeated without rooting out terrorist sanctuaries across the border - a likely reference to Pakistan, where the Taliban and other groups are believed to recruit fighters and base their leadership.
The increasingly outspoken Afghan leader's comments echo a common complaint among President Barack Obama's critics that the deadline gives the Taliban motivation to hold out until after next July and then make a new push for power. Obama himself has stressed that any troop withdrawals will be linked to the security situation, and American military leaders have recently been saying it could take much longer to train Afghan forces.
Violence has spiked around the country as the Taliban push back against a new security push by U.S.-led international force - bolstered by 30,000 U.S. troops in the insurgents' southern and eastern strongholds.
More than 10 militants attacked the police checkpoint outside the northern city of Kunduz, said provincial police chief Abdul Raziq Yaqoubi, adding they suspected the attackers were jihadists from Russia's restive Chechnya region who are active in the surrounding province, also called Kunduz.
He said two or three of the militants were wounded when the police fought back. The militants apparently hoped to steal the policemen's weapons but were beaten back before they could do so, he said.
Kunduz has seen an increasing number of attacks on Afghan and foreign coalition forces who rely on a supply line running south through the province from neighboring Tajikistan. Foreign fighters from Chechnya, Pakistan and the Persian Gulf infiltrate the region from the rugged mountainous border with Pakistan to the east.
Investigations, meanwhile, continued into Wednesday's attack on Spanish troops at a base in the northwestern province of Badghis used by members of Spain's paramilitary Civil Guard to train Afghan police.
Majid Khan Shakib, a member of parliament from Badghis, said the attacker's sister was married to the provincial Taliban commander and the shooting was engineered to incite an uprising against the Spanish. The shooter was killed at the scene by other Spanish police.
After word of the shooting spread, several hundred people protested and hurled stones at the Spanish compound. At least one vehicle was torched and 25 people were wounded by gunshots, although it was unclear who was shooting.
"The Taliban infiltrated the crowd yesterday and agitated everybody. They told people the Spanish were there to colonize the country," Shakib said.
Spain's Interior Ministry initially said the officers' driver opened fire on the men during a training exercise Wednesday. However, Spanish media and Afghan officials said Thursday the shooter was a driver with the Afghan police who occasionally also drove the Spanish officers. He carried an unregistered Kalashnikov rifle to a security checkpoint at the camp entrance and opened fire, provincial police chief Sayed Ahmad Sami said.
NATO spokesman James Appathurai said deliberate killings by Taliban infiltrators were "still very isolated," adding that training Afghan security forces would remain the foundation of a strategy to pass responsibility for security to Afghan forces.
"There are thousands of Afghan army and police being trained every day by NATO soldiers, and it works well. Unfortunately, there are still occasionally incidents like these," Appathurai said.
The string of attacks in the north shows the Taliban and their allies are capable of fomenting instability beyond their traditional strongholds in the east and south, which is the focus of U.S.-led military operations. Provinces in the north previously had been largely spared the violence that have affected provinces such Helmand and Kandahar in the south and Logar, Wardak, Kunar and Khost in the east.
Also Thursday, a candidate in next month's parliamentary elections said 10 of her campaign workers were kidnapped while traveling in the western province of Herat.
Fawzya Galani said she lost contact with the group at about 6 p.m. Wednesday. Villagers told her armed men had stopped the group and driven off in their two vehicles, Galani said.
Local district chief Nisar Ahmad Popal said it wasn't clear whether the kidnappers were political rivals or members of the Taliban, who are seeking to sabotage the Sept. 18 elections for 249 seats in the lower house of parliament.
Citing security concerns, Afghanistan's electoral commission has reduced the number of voting sites for the elections by almost 1,000 to 5,897. It said Thursday that number could drop further if voter safety could not be ensured.
Many Afghans say they plan not to vote, either because of insurgent threats or out of disgust with rampant corruption among government officials.
In eastern Ghazni province's Andar district, two Afghan guards working for a private security company were killed in a Taliban attack on a supply convoy, provincial police chief Zarawar Khan Zahid said.
Two attackers were killed, including a senior regional commander, Mullah Mohmmadi, Zahid said.
NATO has been stepping up operations ahead of the elections and said Thursday it had detained several insurgents in Khost province along the Pakistan border while pursuing senior members of the Haqqani network, an Islamist militant group with deep links to al-Qaida.
The alliance said Afghan and coalition forces captured two Haqqani and several Taliban leaders during 35 separate operations this week.
NATO also reported that three Afghan civilians were killed Wednesday by a homemade bomb in Kandahar's Arghandab district, a Taliban stronghold that has had a growing coalition presence.
Two Taliban commanders were also killed Wednesday in fighting with a joint Afghan-Taliban force in neighboring Uruzgan province, along with 12 regular insurgent fighters, the Afghan National Police reported.
Associated Press Writer Mirwais Khan in Kandahar contributed to this report.