A bomb ripped through a bus in southern Afghanistan on Friday, killing 14 civilians as the Taliban threatened to step up already rising suicide attacks.
The threat came after the Taliban accused Nato forces of genocide after the latest in a series of civilian combat deaths.
"We want to inform the foreign forces and their slaves that their defeat is inevitable in Afghanistan," the Islamist group's one-legged military commander, Mullah Dadullah, told Reuters by satellite phone from a secret location.
Witnesses and officials say Nato air strikes in Kandahar province, where the Taliban began and remain strong, killed at least 50 civilians this week in an area the alliance had said it had cleared of insurgents in a recent offensive.
"The Taliban's mujahideen are ready to fight until death and in the coming days will increase their activities and suicide attacks to such an extent that the infidel forces will not get a chance to rest," Dadullah said.
He denied Nato charges the guerrillas used villagers as human shields in combat against foreign forces, and instead accused the alliance of killing women and children.
"The Taliban will not let the killers of Afghan women and children rest in peace and will continue to target them."
In a separate incident, a provincial official said a bomb had killed at least 14 civilians in the rugged southern province of Uruzgan.
The blast tore through a bus, but officials said it was not clear if it was a suicide attack or a roadside bomb.
TALIBAN VIDEO
The Defence Ministry, Nato and a team of local elders appointed by President Hamid Karzai are investigating this week's air strikes.
"We share solidarity with the families of the victims and the people of Afghanistan are hurt and saddened by this incident," Karzai told reporters on Friday at the presidential palace.
He said the only way to avoid more such deaths was to use Afghan soldiers and police -- who can better spot militants from villagers -- more in operations, build an air force and for the international community to tackle the root causes of terrorism.
Fighting, mainly in the Taliban's southern stronghold, is the worst since US-led forces drove the group from power in 2001.
Recent Taliban video shows a robust Dadullah walking the mountains of Uruzgan and firing a machine gun.
Karzai this week appealed to fellow ethnic Pashtun leaders in neighbouring Pakistan to help quell the Taliban insurgency.
The mainly Pashtun Taliban freely cross the porous border. Afghanistan accuses Pakistan of not doing enough to stop them, or even of continuing to support its former protege.
Islamabad and most Western nations reject this.
Karzai sought help from Maulana Fazal-ur-Rehman, leader of the opposition in the National Assembly and a leading cleric, and Pashtun nationalist leader Asfandayar Wali Khan.
Khan, head of the Pashtun nationalist Awami National Party, said Karzai wrote to him and spoke with him on Thursday.
"Right now two forces are operating in the region. One is promoting war, hatred and isolation while the other is trying for peace and harmony," Khan said. "We are in the latter camp."
Kabul and Islamabad, key US allies in its war on terrorism, last month agreed to call tribal gatherings, or jirgas, on both sides of the border to win support against the Taliban.
As Nato deals with the fallout of the civilian deaths, Germany said on Friday it has suspended two soldiers for their part in the desecration of human skulls in Afghanistan.