In two separate instances, three NATO soldiers have been killed by local security forces in Afghanistan.
In the south, a gunman in an Afghan army uniform killed two British soldiers in a military base in Helmand province, NATO's International Security Assistance Force (ISAF) has said.
Monday's late morning attack in Lashkar Gah, the capital of the southern province, took place at the main entrance to a base housing military and civilian reconstruction teams, the provincial governor's office said in a statement.
Afghan and Western security sources identified the two NATO soldiers as British troops. An Afghan soldier was also shot dead and one Briton was wounded in the attack according to a Western security source speaking on condition of anonymity to the AFP news agency.
Provincial police chief Abdul Nabi Elham said the gunman was a lieutenant named Gul Nazar from the eastern city of Jalalabad.
"As soon as the NATO soldiers opened the ga[t]e for him and his team in their centre, this soldier opened fire at them and killed them," he said.
"We don't know the motive behind this attack and have not found a link to the Taliban. We are still investigating."
Reports of an argument ahead of the shootings have yet to be substantiated.
In a separate instance in the east, a NATO service member was shot by an alleged member of the Afghan police when approaching a security check point.
A senior US military official speaking to US media has said that the service member was an American.
Green on blue
The deaths are the latest in a series of "green-on-blue" killings, where Afghan security forces turn their guns on their international colleagues, an ISAF spokesman said.
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Fifteen personnel - six Americans, four French army trainers, an Albanian and two ISAF personnel whose nationality has not been disclosed - from NATO's US-led ISAF have died in such incidents this year, the spokesman said.
Monday's deaths bring the number of NATO forces killed in the 42 insider attacks from May 2007 to January of this year to 70.
Al Jazeera's James Bays, reporting from the Afghan capital Kabul, said "The big question is are [the attackers] acting on their own, or are they evidence of widespread Taliban infiltration?"
An Afghan army general has said that the Taliban have a sophisticated system to infiltrate Afghanistan's security forces and vetting of recruits must be severely tightened.
These insider attacks are "very, very worrying for the international forces and for the Afghan government, because over the next year or so they are supposed to be transferring and transitioning control to the Afghans", our correspondent added.
General John Allen, commander of NATO forces in Afghanistan, speaking to Pentagon reporters in Washington on Monday said he expects Afghan soldier attacks on foreign forces to continue.
The Lashkar Gah base is dominated by British and US forces, although there are also some Danish and Estonian troops stationed there, Bays said.
"A joint Afghan and ISAF team is investigating the incident," said Captain Justin M Brockhoff, a spokesman for ISAF.
Elsewhere in the country, a suicide bomber detonated an explosive-ridden motorcylce in the central province of Uruzgan.
The noon-time attack targeted an ISAF military base in the district of Choora.
As a convoy was exiting the main bazaar in the provincial capital of Tirinkot, the bomber attacked , a witness told Afghan media. Afghan and foreign forces cordoned off the area after the explosion.
An ISAF statement said the patrol suffered no casualties. A security official speaking to Afghan media said three ISAF soldiers and one Afghan policeman were wounded in the attack.
The local governor has confirmed the three NATO service members as Australian soldiers.
The Taliban, known to exaggerate in their reporting of casualties and impact of the attacks for which they claim responsibility, have released a statement on their website saying the Uruzgan attack is revenge for the "massacre" linked to US Staff Sergeant Robert Bales.
"Local Mujahideen leaders say that this opeation was carried out to avenge the 17 martyrs of Zangawat (Kandahar) who were brutely shot and then some burnt by the cowardly American terrorists in a night-time massacre a few days ago" the statement reads.
Source: Al Jazeera and agencies