Nigerian villagers who are suing Chevron Corp. in a San Francisco federal court have quietly moved to withdraw half of their claims that the oil company was responsible for military attacks on protesters in the late 1990s.

In papers filed last week, plaintiffs' lawyers, without explanation, asked a federal judge to dismiss claims by 25 Nigerians over a January 1999 attack on villages near oil facilities in the Niger Delta where residents had protested pollution. In their lawsuit, the plaintiffs had said government troops, using a helicopter and boats supplied by Chevron, killed at least four unarmed people and burned two villages to the ground.

The dismissal does not affect the rest of the case, which is scheduled for trial in September. It involves a May 1998 incident in which military police opened fire on a Chevron Nigeria offshore oil rig that had been occupied by more than 100 people who said the company had polluted their land and water and denied employment to their people.