JUBA, Sudan (Reuters) - At least 20 soldiers were killed in a clash between south Sudan's army and rebel militia fighters, the army said on Wednesday, the latest violence to unsettle the region ahead of its independence in July. Sudan's oil-producing south voted to separate from the north in a January referendum promised under a 2005 peace accord that ended decades of civil war. The underdeveloped region, the source of most of Sudan's 500,000 barrels per day of oil, has been beset by violence since the vote.