Ideally, one American commander said, the stockpiles would be surrounded, protected and then incinerated, much as the United States has done with its chemical arsenal. But that takes years, and as one official said, “We don’t have years, and we can’t keep troops there.”

That is why attacking the delivery systems seems like the next best option to many in the administration. Israel was believed to be behind an attack on some Syrian missiles in January as they were about to be transported, presumably to Hezbollah. On Wednesday, Prime MinisterBenjamin Netanyahu told Israeli lawmakers that a Hezbollah missile attack, using chemical weapons, was one of his chief concerns.

If Mr. Obama and his allies proceeded with an attack on air defenses, missiles and the Syrian Air Force, they would most likely use Tomahawk cruise missiles launched from ships in the eastern Mediterranean and fighter jets that might be able to fire missiles without entering Syrian airspace. But it is unclear how effective those would be.

Mr. Obama has always made clear that any action should be taken with allies and neighbors. But NATO has been reluctant, and Russia, which keeps a naval base in Syria, has been opposed. Israeli officials have said that they do not want to go into Syria, fearing that any Israeli attack would fuel Mr. Assad’s argument that the civil war in his country is the result of foreign provocations. Some Israeli officials have argued that the Arab League should be in the vanguard of any attack, but it has shown little interest in direct military intervention in the Syrian conflict.

That has left the same trio that led the attack on Libya in 2011: the United States, Britain and France. There has been constant discussion among their militaries about “options of every kind,” one official involved in the talks said this week. “Clearly, an airstrike would be much more complex than in Libya,” the official said, noting that most of the targets there were in the desert.

The deliberations on how to respond militarily to any confirmed use of chemical weapons was taking place against the backdrop of some of the most intense conventional fighting in the two-year-old Syrian conflict, which has left more than 70,000 people dead.

Opposition activists and fighters in Syria accused Mr. Assad’s military of carrying out attacks for the second straight day on the Mediterranean seaport of Baniyas and the village of Bayda, where dozens of civilians, including children, were found dead Thursday, some stabbed and burned. The National Coalition of Syrian Revolutionary and Opposition Forces, the main anti-Assad political group, said in a statement that the attacks constituted another war crime.

Syria’s official SANA news agency said nothing about civilian killings in Baniyas or Bayda in its dispatches on the fighting, asserting that its forces had “destroyed a number of terrorists’ dens and gatherings in several areas, killing and injuring many terrorists.” It also said insurgents had lobbed mortar shells at the Damascus airport.