Authorities in Afghanistan say they are searching for an Afghan soldier who killed three British troops in the southern part of the country.
Officials say the Afghan soldier attacked British troops in central Helmand province during a joint patrol.
Lieutenant General Nick Parker, the deputy commander of international forces in Afghanistan, says the incident appears disturbingly familiar.
"It does look to us as if this is another example of a member of Afghan National Security forces attacking our soldiers," Parker said.
Last November, an Afghan police officer shot dead five British soldiers at a training base in Helmand province.
A few weeks later, an Afghan soldier killed a U.S. service member and wounded two Italians at a base in the country's northwest.
Parker says the attacks are a "serious breach of trust" that could have lingering consequences.
"When your partner, the one you are fighting with, turns on you, it has a real effect on you, and we should not try to understate that," Parker said.
In Kabul, Afghan presidential spokesman Waheed Omar says the Defense Ministry is investigating the latest incident.
"If this is true, then it is a very regrettable case," Omar said. "And we hope that it is totally investigated and that we have good information as a result of the investigation."
Officials say Afghan President Hamid Karzai also has sent a letter of apology to NATO and the British government.
Violence has increased in Afghanistan as NATO and Afghan troops intensify military efforts to tackle Taliban strongholds in Helmand and neighboring Kandahar province.
Last month was the deadliest for international forces since 2001, with more than 100 NATO troops killed.