At a White House news conference, the president also defended the FBI's work in monitoring the activities in recent years of one of the men accused in the deadly bombing at the Boston Marathon two weeks ago.
At a question and answer session that ranged from immigration legislation to recent intelligence cooperation with Russia, the president several times chided, criticized or dismissed his Republican critics. Asked about one senator who recently said national security protections have deteriorated since he became president, Obama said, Sen. Lindsey "Graham is not right on this issue, although I'm sure he generated some headlines."
Asked about Syria, the president said that while there is evidence that chemical weapons were used inside the country, "we don't know when they were used, how they were used. We don't know who used them. We don't have a chain of custody that establishes" exactly what happened.
If it can be established that the Syrian government used chemical weapons, he added, "we would have to rethink the range of options that are available to us."
"Obviously there are options to me that are on the shelf right now that we have not deployed," he said, noting that he had asked Pentagon planners last year for additional possibilities. He declined to provide details.
Obama responded with slight ridicule and humor when he was asked if he still had the political juice to push his agenda through Congress after an early second-term defeat on gun control legislation.
"Golly, I might just as well pack up and go home," he parried his questioner. Paraphrasing Mark Twain, he said, "Rumors of my demise may be a little exaggerated at this point." He expressed confidence that Congress would approve sweeping immigration legislation that he is seeking.
He also renewed his call for lawmakers to replace across-the-board federal spending cuts. The administration favors a comprehensive plan to reduce deficits through targeted spending cuts and higher taxes.
Asked about the FBI's investigation into a possible terrorist threat posed in the past by Tamerlan Tsarnaev, a suspect in the Boston Marathon bombings who died in an escape attempt, the president said, "Based on what I've seen so far, the FBI performed its duties , the Department of Homeland Security did what it was supposed to be doing."
"But this is hard stuff," he said of the work needed to ferret out security threats at home.
He also said that "Russians have been very cooperative with us since the Boston bombing."
The bombing suspects are Russian natives who immigrated to the Boston area. Russian authorities told U.S. officials before the bombings they had concerns about the family, but Moscow has revealed details of wiretapped conversations only since the attack.
Obama had scarcely completed his news conference when Graham, the Republican South Carolina senator, responded to his comment about national security.
"With all due respect, Mr. President, Benghazi and Boston are compelling examples of how our national security systems have deteriorated on your watch." He referred to the attack that killed three Americans at a U.S. diplomatic post in Libya last year, as well as the marathon bombing
Asked about a topic that links terrorism and his Obama's legislative efforts, he said he would "re-engage with Congress" on his goal of closing the prison for detainees at Guantanamo in Cuba. As a candidate for the White House in 2007 and 2008, Obama called for closing the base, which was set up as part of President George W. Bush's response to the terror attacks on Sept. 11, 2001. Lawmakers objected and the facility remains open.
Asked about a hunger strike by some detainees, he said, "I don't want these individuals to die," and he said the Pentagon was doing what it could to manage the situation.
Obama also noted that several suspected terrorists have been tried and found in U.S. federal courts, an answer to his congressional critics who maintain that detainees must be tried in special courts if the United States is to maximize its ability to prevent future attacks.
On another contentious issue, the president said a variety of Republicans was working to foil the final implementation of the health care law he pushed through Congress three years ago.
He said GOP lawmakers want to repeal the law and some Republican governors don't want to have their states participate in establishing insurance pools where the uninsured can find coverage. In other cases, Republican legislatures object when governors are willing to go along.
Even so, he said, "we will implement" the law, although he conceded there will be glitches along the way.
"Despite all the hue and cry and sky-is-falling predictions about this stuff, if you've already got health insurance, then the part of Obamacare that affects you is already in place," he added.
The first question to Obama concerned Syria and the reported use of chemical weapons.
Administration officials said recently that intelligence analysts had "varying degrees of confidence" in a conclusion that Assad's government has deployed sarin gas against civilians.
Obama said the administration was using all its resources to determine the facts about a weapon that he has said would be a "game changer" for U.S. policy in the war.
"If we end up rushing to judgment without hard, effective evidence ... we can find ourselves in a position where we can't marshal the international community in support of what we do," he said. "It's important for us to do this in a prudent way."
He did not say so, but one of the enduring controversies from Bush's administration was the assertion - never proven, but used to justify the invasion of Iraq - that Saddam Hussein possessed weapons of mass destruction.
The Obama administration long ago called for Assad to step down and pave the way for a new government, but Obama has resisted calls from some Republicans in Congress to send U.S. military aid to the rebels or commit U.S. military resources directly.
The hour-long news conference concluded with a post-script.
Obama had stepped away from the lectern when he heard a shouted question about Jason Collins, the professional basketball player who made a pioneering announcement on Monday that he is gay.
Obama said he had spoken with Collins and "told him I couldn't be prouder of him."
"One of the extraordinary measures of progress that we've seen in this country has been the recognition that the LGBT community deserves full equality, not just partial equality, not just tolerance but a recognition that they're wholly a part of the American family," he said.