WASHINGTON —  Obama has directed the Pentagon to develop an expanded list of potential targets in Syria in response to intelligence suggesting that the government of President Bashar al-Assad has been moving troops and equipment used to employ chemical weapons while Congress debates whether to authorize military action.

Mr. Obama, officials said, is now determined to put more emphasis on the “degrade” part of what the administration has said is the goal of a military strike against Syria — to “deter and degrade” Mr. Assad’s ability to use chemical weapons. That means expanding beyond the 50 or so major sites that were part of the original target list developed with French forces before Mr. Obama delayed action on Saturday to seek Congressional approval of his plan.

For the first time, the administration is talking about using American and French aircraft to conduct strikes on specific targets, in addition to ship-launched Tomahawk cruise missiles. There is a renewed push to get other NATO forces involved.

The strikes would be aimed not at the chemical stockpiles themselves — risking a potential catastrophe — but rather the military units that have stored and prepared the chemical weapons and carried the attacks against Syrian rebels, as well as the headquarters overseeing the effort, and the rockets and artillery that have launched the attacks, military officials said Thursday.

Gen. Martin E. Dempsey, the chairman of the Joint Chiefs of Staff, has said that other targets would include equipment that Syria uses to protect the chemicals — air defenses, long-range missiles and rockets, which can also deliver the weapons.

Mr. Obama’s instructions come as most members of Congress who are even willing to consider voting in favor of a military response to a chemical attack are insisting on strict limits on the duration and type of the strikes carried out by the United States, while a small number of Republicans are telling the White House that the current plans are not muscular enough to destabilize the Assad government.

Senior officials are aware of the competing imperatives they now confront — that to win even the fight on Capitol Hill, they will have to accept restrictions on the military response, and in order to make the strike meaningful they must expand its scope.

“They are being pulled in two different directions,” a senior foreign official involved in the discussions said Thursday. “The worst outcome would be to come out of this bruising battle with Congress and conduct a military action that made little difference.”

Officials cautioned that the options for an increased American strike would still be limited — “think incremental increase, not exponential,” said one official — but would be intended to inflict significant damage on the Syrian military.

It was a measure of the White House’s concern about obtaining Congressional approval that Mr. Obama canceled a planned trip to Los Angeles next week, where he was scheduled to speak to the A.F.L.-C.I.O. and hold a fund-raiser. One senior official said Mr. Obama would get far more involved in direct lobbying for a military authorization, and there is talk inside the administration about a formal address to the nation.

In endorsing a strike on Wednesday, the Senate Foreign Relations Committee made some modifications to the resolution proposed by the White House, and other versions are also being circulated. The latest is from Senator Joe Manchin III, a conservative Democrat from West Virginia who proposes giving Mr. Assad 45 days to sign the Chemical Weapons Convention and begin securing and ridding the country of its weapons stockpiles. Only if Mr. Assad refuses would the president be authorized to take military action.

“We need some options out there that does something about the chemical weapons,” Mr. Manchin said. “That’s what’s missing right now.”

The concept is already being debated by some government officials and foreign diplomats, though the White House has not weighed in.

For now, White House officials insist that they are slowly gaining ground in lining up support, though the evidence is slim. “We’re very pleased with the trend lines,” said Benjamin J. Rhodes, the president’s deputy national security adviser. “I think each day what you’ve seen is different members coming out on a bipartisan basis to support an authorization to use military force.”

He noted Wednesday’s Senate committee vote and the endorsements from a range of senators, including from John McCain, Republican of Arizona, and the liberal Democrat Barbara Boxer of California. “What we’re seeing each day is an increasing number of members who are convinced that a military response is necessary,” Mr. Rhodes said. “But we’re going to continue to make the case to members.”

Privately, some members of the Obama administration appear concerned that General Dempsey’s presentations to Congress — particularly his repeated assertions that any American intervention in Syria is unlikely to have a decisive effect on the civil war — are undercutting the administration’s argument that the attacks, while targeted, would also change Mr. Assad’s calculus.

So as the target list expands, the administration is creeping closer to carrying out military action that also could help tip the balance on the ground, even as the administration argues that that is not the primary intent.

The bulk of the American attack is still expected to be carried out by cruise missiles from some or all of the four Arleigh Burke-class destroyers within striking range of Syria in the eastern Mediterranean. Each ship carries about three dozen Tomahawk cruise missiles, a low-flying, highly accurate weapon that can be launched from safe distances of up to about 1,000 miles.

But military planners are now preparing options to include attacks from Air Force bombers, a development reported Thursday by The Wall Street Journal. The Pentagon was initially planning to rely solely on cruise missiles.

Bombers could carry scores more munitions, potentially permitting the United States to carry out more strikes if the first wave does not destroy the targets.

Among the options available are B-52 bombers, which can carry air-launched cruise missiles; B-1s that are based in Qatar and carry long-range, air-to-surface missiles; and B-2 stealth bombers, which are based in Missouri and carry satellite-guided bombs.

The Navy in recent days has moved the aircraft carrier Nimitz into the Red Sea, within striking distance of Syria.

But Defense Department officials said Thursday that the Nimitz, and its squadrons of F-18 Super Hornet attack planes, as well as three missile-toting destroyers in its battle group, are not likely to join any attack unless Syria launches major retaliatory strikes.

Defense Secretary Chuck Hagel told lawmakers on Wednesday that an American operation would cost “in the tens of millions of dollars,” the first time any administration official has put even a rough price tag on the possible strike.

Mr. Assad has openly mocked the United States for delaying any military action, and has seized on the pause to move military equipment, troops and documents to civilian neighborhoods, presumably daring Mr. Obama to order strikes that could kill large number of civilians.

“The additional time gives Assad the potential advantage of complicating U.S. targeting by surreptitiously moving people or even chemical munitions into them, aiming to create casualties or chemical release as a direct result of U.S. attacks,” said David A. Deptula, a retired three-star Air Force general who planned the American air campaigns in 2001 in Afghanistan and in the 1991 Persian Gulf war.

But General Dempsey told lawmakers on Wednesday that American spy agencies are “keeping up with that movement,” which he said also included prisoners, potentially to be used as human shields, and that all necessary steps would be taken to minimize civilian casualties.

The Pentagon is also planning contingencies to counter or respond to any retaliatory attacks by Mr. Assad’s forces. General Dempsey said the Syrian leader could lob long-range rockets against his neighbors; encourage surrogates and proxies, like Hezbollah, to assault American embassies; or carry out a cyberattack against the United States or American interests.

“We are alert to all of the possibilities and are mitigating strategies in the way we’ve positioned ourselves in the region,” General Dempsey said.