LONDON — Prospects for a Western-led military strike on Syria increased on Tuesday as the American defense secretary said United States forces were ready for any contingency, the British and French leaders spoke with an aggressive new tone and the Arab League joined in accusing the Syrian government of a mass killing last week with a chemical munitions attack.

The developments came as United Nations weapons inspectors in Syria postponed a second visit to suspected attack sites on the outskirts of the capital, Damascus, after having failed to secure assurances of their safety, the United Nations and Syrian officials said.

Even without the evidence that the inspectors are collecting, the United States and other Western powers have concluded that the Aug. 21 attack, which killed hundreds of people, was caused by banned chemical munitions and that President Bashar al-Assad’s forces were responsible, crossing a threshold that required a forceful response.

Chuck Hagel, the United States secretary of defense, said in an interview with the BBC that American forces had “moved assets in place to be able to fulfill and comply with whatever option the president wishes to take.” Asked how soon these forces could be ready, Mr. Hagel said, “We are ready to go.”