KABUL - A suicide bomber penetrated a foreign army base in Afghanistan and killed eight CIA employees on Wednesday, one of the U.S. agency's largest death tolls, while four Canadian troops and a journalist died in a separate attack.
The attack is one of the most ambitious of the war, highlighting the insurgency's reach and coordination at a time when violence has reached its highest levels since the overthrow of the Taliban regime by U.S.-backed Afghan forces in 2001.
The Taliban claimed the attacker as a sympathiser from the Afghan army, but the country's Defence Ministry said no Afghan soldiers were involved in the attack or stationed at the site in southeastern Khost province.
U.S. officials said the dead Americans were CIA employees but declined to comment on the attacker's nationality or status. NATO-led forces said the base was not under their control.
But either way the explosion casts a long shadow over Western plans to bolster the Afghan army and police, in order to eventually hand over security and bring their own troops home.
If an Afghan army official turned on the foreign troops and officials who are meant to be mentoring and partnering them, after a series of similar incidents this year, it will raise tough questions about trust and loyalty.
If he was not an Afghan soldier visiting or stationed on Forward Operating Base Chapman, then U.S. concerns about security controls will be even more grave, after a year that has been the deadliest of the eight-year war for foreign troops.
U.S. President Barack Obama is sending 30,000 extra troops to tackle the mounting violence and NATO allies are contributing thousands more. An Afghan army official said this week Washington had pledged $16 billion (9.9 billion pounds) to train the army and air force.
The CIA has also been expanding its presence in the country, stepping up strikes against Taliban and al Qaeda militants along the border between Afghanistan and Pakistan, although its role has been criticised by rights groups and Afghans.
The site of the suicide attack is near the Pakistan border, in one of the areas where the Taliban insurgency is strongest. Security in the region was stepped up on Thursday.
Asked how the attacker launched the assault in a foreign military base, Taliban spokesman Zabihullah Mujahid replied: Since the man was an officer, he had not much difficulties.
The five Canadians -- four soldiers and a journalist -- were killed when their armoured vehicle was hit by a bomb in southern Kandahar province, the Canadian Defence Ministry said.
The blast, about 4 km (2.5 miles) outside Kandahar, struck the patrol as it was visiting community reconstruction projects.
Washington has pledged a civilian surge, adding hundreds of U.S. experts to support work on development projects that aim to undermine support for the Taliban and other insurgents.
But foreign aid agencies warned earlier this year that the shift into the military bases, and the use of military personnel to carry out development projects, risked a dangerous blurring of the boundaries between troops and civilians.
The journalist killed was Michelle Lang, 34, on assignment for the Canwest News Service. She was on her first assignment in Afghanistan and had been in the country since December 11.
She is the third journalist to die in Afghanistan this year.
In another incident, a number of civilians were killed by a foreign forces' air strike in southern Helmand province on Wednesday, Daoud Ahmadi, the provincial governor's spokesman, said. He did not know how many had died.
A patrol of foreign troops came under Taliban ambush at 3 p.m. After the ambush, planes came and bombed the area, which caused civilian casualties, he said. An investigation is already under way at the site. A press officer for NATO-led forces declined immediate comment on the incident.
The incident is the second report of civilian casualties at the hands of foreign forces in less than a week, and has heightened tensions between the Afghan government and foreign troops.
Afghan officials say foreign troops killed 10 civilians, mostly teenagers, in a raid in eastern Afghanistan on Saturday, sparking street protests around the country.
The NATO-led force said it was questioning the claims of civilian casualties and maintains they were insurgents. It has called for a joint investigation with Afghan authorities.
The news of more casualties came on Thursday as hundreds of people protested in Kunar, where Saturday's raid took place.
(Additional reporting by Ismail Sameem in KANDAHAR; Writing by Emma Graham-Harrison; Editing by Nick Macfie)