Two Afghan police officers have killed nine of their colleagues after opening fire on them, while two attacks on NATO-led troops in Helmand province have left six service members dead, officials said.
The police officers, whose attackers were killed, were fatally shot on Saturday in Delaram district of western Nimroz province, the latest incident in a series of shootings in which members of the Afghan security forces have targeted local and foreign colleagues.
"Unfortunately 11 of our brave national police were killed in this cowardly attack," Abdul Karim Brahawi, Nimroz governor, told the AFP news agency.
"Initial investigation shows the shooter was a Taliban infiltrator. He was also killed when police returned fire," Brahawi said.
Abdul Majid Latifi, provincial deputy police chief, confirmed that 11 police had been killed by "a man in police uniform", adding that an investigation into the attack was under way.
On Friday, an Afghan working on an installation shared by Afghan and foreign forces shot and killed three foreign soldiers, the NATO-led military coalition has said.
On the same day, an Afghan police officer shot three US marines after inviting them to dinner.
The officer shot and killed the three marines after sharing a meal with them before dawn in Sangin district of Helmand province.
"Asadullah, the police checkpost commander, invited four foreign special forces soldiers to a [Ramadan] breakfast at 2:30 am in Sangin district," a senior security officer in Helmand said.
"He later opened fire on the special forces soldiers, killing three and wounding another."
Qari Yousuf Ahmadi, a Taliban spokesman, speaking to AFP by phone said: "Asadullah joined the mujahideen ranks after the killing."
Major Lori Hodge, a spokeswoman for the coalition, said the shooters in both attacks had been detained.
NATO has about 130,000 soldiers helping the Afghan government fight the Taliban, but they are due to pull out in 2014 and are increasingly working with Afghans they are training to take over.
"Clearly as far as the future partnering and training and mentoring of Afghan forces by NATO and the US is concerned, it is going to have a very negative effect and the lack of trust between the two sides is going to grow," Ahmed Rashid, an author and analyst, said.
"NATO will have to impose new security measures for its own troops when they are dealing with Afghans or training Afghans, which will put even more distance between the two sides," Rashid told AFP.
Some of the attacks are claimed by the Taliban, who say they have infiltrated the ranks of Afghan security forces, but many are attributed to cultural differences and antagonism between local and coalition forces.
"What we identified was that most of them were caused by personal grievances and stress situations," the chief spokesman for NATO's International Security Assistance Force, Brigadier-General Gunter Katz, told AFP.
"Those isolated incidents don't reflect the overall security situation in Afghanistan. As we speak 500,000 soldiers and policemen are working together to contribute to a more secure and stable Afghanistan.
"We are confident that the morale [among international troops] is still good and those incidents will not affect our transition process."
Katz agreed, however, that there had been an increase in so-called green-on-blue attacks this year in which Afghans turn their weapons against their NATO allies.
NATO had recorded 26 incidents in which 34 international soldiers were killed, he said.