By Rod Mcguirk THE ASSOCIATED PRESS MELBOURNE, Australia (AP) -A tribal clash in Papua New Guinea’s remote highlands in which at least 26 people were killed has put a growing internal security problem under the microscope in the strategically vital South Pacific island nation that has garnered closer military attention from the United States and China. The rival powers are increasingly keen to strike security pacts with the developing nation whose international significance has grown in tandem with China’s increased regional influence. Here’s a look at some key issues surrounding Papua New Guinea’s internal security concerns: PAPUA NEW GUINEA, THE COUNTRY Papua New Guinea is the most populous South Pacific island nation after Australia and was a colony of that near neighbor until independence in 1975. The government estimates Papua…