In shootings not previously reported by American authorities, two U.S. soldiers were wounded in an attack by two Afghan police officers, extending a worrying pattern of violent clashes between supposed allies.

The two Americans were shot Wednesday by the Afghans near Kandahar city, and the shooters were quickly killed by U.S. troops, according to a U.S. defense official. The official spoke on condition of anonymity to discuss an incident that, by policy, the coalition military command in Kabul considers an internal matter.

Spokesmen for the U.S.-led international military coalition in Afghanistan have said their policy is to publicly report only those incidents of Afghan forces shooting at coalition forces if a coalition member is killed. If coalition troops are wounded but not killed, or if the Afghan shooter fails to hit anyone, the incident is not disclosed.

About two hours after the two U.S. soldiers were wounded, a separate clash occurred in which an Afghan soldier opened fire with a machine gun on a group of U.S. and Afghan special operations soldiers returning to their base after conducting an operation, U.S. officials said.

The gunfire killed one American soldier, wounded three other Americans and killed another person whose nationality was unclear. One official said the person was an Afghan interpreter. Another official said an internal report referred to him as a U.S. civilian.

That incident, elsewhere in Kandahar province, was reported in spare terms by the international military coalition on Thursday. It said only that one coalition service member was killed and that the Afghan shooter was subsequently killed. The report made no mention of the third death or of the wounding of three American soldiers.

Thus in the space of about two hours Wednesday, two incidents of Afghans turning their guns on Americans resulted in five people dead and five wounded. The Taliban were less deadly that day, killing one American.

The problem of Afghan forces attacking American and other coalition troops has grown more troubling in recent months.

In the first four months of this year there have been 11 such fatal attacks by Afghans, resulting in 18 deaths, according to James Graybeal, a public affairs officer for the international coalition. At that pace, the number killed for the whole year would be about 50 percent greater than in 2011, which itself saw an increase in such attacks.

A Pentagon spokesman, Navy Capt. John Kirby, said that while calculations are imprecise it is believed that 30 percent to 40 percent of the attacks are by Taliban sympathizers or Afghans associated with the Taliban. The rest are believed to be motivated by personal grudges or other grievances against the coalition.

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Robert Burns can be followed on Twitter at http://twitter.com/robertburnsAP