* Agrees to release seven detainees
* Peace talks resume Feb. 7, grievances deep
* Fighting hit oil areas in world's newest state
By Carl Odera and Andrew Green
JUBA, Jan 28 (Reuters) - South Sudanese rebel leader Riek Machar and six others should be tried for treason for their role in weeks of bloodshed, the justice minister said on Tuesday, threatening to heighten tensions in already ...