WASHINGTON — The prospect of a deadly siege of the rebel stronghold in Benghazi, Libya, has produced a striking shift in tone from the Obama administration, which is now pushing for the United Nations to authorize aerial bombing of Libyan tanks and heavy artillery to try to halt the advance of forces loyal to Col. Muammar el-Qaddafi.

The administration, which remains deeply reluctant to be drawn into an armed conflict in yet another Muslim country, is nevertheless backing a resolution in the Security Council that would give countries a broad range of options for aiding the Libyan rebels, including military steps that go well beyond a no-flight zone.

Administration officials — who have been debating a no-flight zone for weeks — concluded that such a step now would be “too little, too late” for rebels who have been pushed back to Benghazi. That suggests more aggressive measures, which some military analysts have called a no-drive zone, to prevent Colonel Qaddafi from moving tanks and artillery into Benghazi.

The United States is insisting that any military action would have to be carried out by an international coalition, including Libya’s Arab neighbors.