Even if the strikes are somewhat limited, the official asserted, they would discourage the Assad government from again using chemical weapons and that, in any case, the condition of Syrian forces would weaken over time.

But he suggested that the push to improve the rebels’ fortunes on the battlefield, and ultimately foster the condition for a possible political settlement, would depend more on increasing efforts to arm the opposition and improve its ability to govern the area it has captured.

The official introduced some of the arguments American officials are expected to make to their European counterparts during Mr. Kerry’s four-day trip. He asserted that the failure to act militarily presented graver risks than a limited military intervention.

Syrian civilians, fearful that the Assad government would strike again with chemical weapons, would continue to flood into Lebanon, Jordan and Iraq, he said. Moderate elements of the Syrian opposition would be weakened.

Extremists who have argued that the United States cares only about Israel and that it is foolhardy to rely assurances from the White House, he said, would get a “boost.”

Mr. Kerry’s trip was initially designed mainly to support the peace talks between Israel and the Palestinians, which resumed after nearly three years. He plans to meet in London on Monday with Mahmoud Abbas, the president of the Palestinian Authority. He had planned to meet in Rome with Prime Minister Benjamin Netanyahu of Israel, but with tensions running high on Syria the Israeli decided to remain in Jerusalem.

A senior official who works on Middle East issues said that the United States was prepared to propose ideas to bridge differences between the two sides but that such efforts, which could include a three-way meeting of Mr. Kerry, Mr. Abbas and Mr. Netanyahu, would come later in the talks.