The Obama administration hardened its stance against Syria and stepped up plans for possible military action, dismissing as too late the regime's offer to let United Nations officials inspect areas where the U.S. believes Damascus used chemical weapons last week.

The White House and Pentagon signaled the U.S. wasn't backing away from a possible showdown despite apparent efforts by the Syrian government to ease tensions by letting U.N. inspectors visit areas near the capital where hundreds were killed, allegedly by chemical weapons.

If he decides to act militarily, Mr. Obama would prefer to do so with U.N. Security Council backing, but officials said he could decide to work instead with international partners such as the North Atlantic Treaty Organization or the Arab League.

"We'll consult with the U.N. They're an important avenue. But they're not the only avenue," a senior administration official said.

In recent days, the Pentagon has moved more warships into place in the eastern Mediterranean and U.S. war planners have updated military options that include cruise-missile strikes on regime targets, officials said. The White House held high-level meetings over the weekend, but officials said late Sunday that Mr. Obama had yet to decide how to proceed.