BERKELEY – UC Berkeley officials have not ruled out forcibly removing four tree-sitters protesting the university's proposed new sports center.

University spokesman Dan Mogulof said Sunday the university was trying to end the standoff peacefully. But he added that “the clock is ticking.”

Tree-sitters have occupied a disputed grove of trees for about 21 months as opponents of the planned $125 million sports center sued to block the project.

The university began cutting down trees Friday, a day after a state appeals court denied a request from two citizens groups for an injunction barring construction.

The trees are now nearly all gone. Attempts to bring the tree-sitters down voluntarily Sunday, including a discussion with UC Berkeley police, were unsuccessful.