Tensions in the administration over the Gulf Coast oil spill escalated Thursday as President Barack Obama prepared to go before reporters to answer questions about the ongoing debacle.

Elizabeth Birnbaum, the head of the Minerals Management Service, resigned Thursday amid criticism of the agency’s lax oversight over offshore drilling operations. She sought to distance herself from the controversy surrounding the MMS in her resignation letter.

“I’m hopeful that the reforms that the secretary and the administration are undertaking will resolve the flaws in the current system that I inherited,” Birnbaum said.

Interior Secretary Ken Salazar called Birnbaum “a good public servant” and said she resigned “on her own volition.”

News of Birnbaum’s departure came just before new government estimates show that the Gulf Coast oil spill is now the worst in U.S. history. BP employed a new tactic to try to stem the leak, called “top kill,” but it was unclear whether the effort would be successful.

Obama is stepping up his efforts this week to appear more engaged in the cleanup efforts for the massive spill, which has been gushing into the Gulf since the BP oil rig exploded April 20. He is holding a White House press conference at 12:45 p.m. to unveil new offshore oil drilling regulations that include more stringent safety inspections and permitting requirements. On Friday, he is flying to Louisiana for the second time to assess the situation.

Adding to the administration’s offensive, Adm. Thad Allen, the national incident commander overseeing the cleanup effort, is holding a conference call Thursday afternoon to give an update on operations.

In the meantime, lawmakers in both parties have been grumbling about the need for more action from the administration. And in the absence of a clear federal plan for preventing future spills of this magnitude, Sen. Bernie Sanders (I-Vt.) on Thursday introduced legislation that would ban all offshore drilling.