U.S. President Barack Obama signaled Tuesday he would consider U.S. military action against Syria if "hard, effective evidence" is found to bolster intelligence that chemical weapons have been used in the two-year-old civil war. Among the potential options being readied for him: weapons and ammunition for the Syrian rebels.

Despite such planning, Obama appealed for patience during a White House news conference, saying he needed more conclusive evidence about how and when chemical weapons detected by U.S. intelligence agencies were used and who deployed them. If those questions can be answered, Obama said he would consider actions the Pentagon and intelligence community have prepared for him in the event Syria has crossed his chemical weapons "red line."

"There are options that are available to me that are on the shelf right now that we have not deployed," he told reporters packed into the White House briefing room.

Beyond lethal aid to the rebels, several government agencies are also drafting plans for establishing a protective "no-fly zone" over Syria and for targeted missile strikes, according to officials familiar with the planning. However, the officials, who spoke only on condition of anonymity because they were not authorized to publicly discuss the internal deliberations, stressed that Obama had not yet decided to proceed on any of the plans.

Hezbollah stands ready to aid Assad

As Obama raised the prospect of deeper U.S. involvement, Hezbollah's leader said Tuesday that his Iranian-backed militant group stood ready to aid Syrian President Bashar al-Assad. And new violence in Syria hit the capital of Damascus, as a powerful bomb ripped through a bustling commercial district, killing at least 14 people.

Mindful that any military intervention in the combustible Middle East would be complicated and dangerous, Obama hinted the U.S. would probably avoid taking action unilaterally. Part of the rationale for building a stronger chemical weapons case against Assad, Obama said, is to avoid being in a position "where we can't mobilize the international community to support what we do."

Obama has resisted calls to expand U.S. assistance beyond the nonlethal aid the government is providing the rebels. That has frustrated some allies as well as some U.S. lawmakers, who say the deaths of 70,000 Syrians should warrant a more robust American response.

Security personnel walk in front of the former Syrian Interior Ministry building after a deadly blast at Marjeh Square in Damascus. (Khaled al-Hariri/Reuters)

Tuesday's wide-ranging news conference coincided with the 100-day mark of Obama's second term. It's a stretch that has been defined by the defeat of gun control legislation he supported, as well as the continuation of old disputes that marked the president's first four years in office, including the Syria conflict and the launching of his controversial health care overhaul. Asked if he still had "the juice" to get legislation approved, he smiled and paraphrased Mark Twain's famous line, saying, "Rumors of my demise may be a little exaggerated at this point."

Another issue that frustrated Obama in his first term resurfaced when he was pressed about the hunger strike at Guantanamo Bay, the detention centre he promised to close but hasn't been able to. Obama said he would make another run at it, though he was vague about how.

"I'm going to go back at this," he said. "I've asked my team to review everything that's currently being done in Guantanamo, everything that we can do administratively, and I'm going to re-engage with Congress to try to make the case that this is not something that's in the best interest of the American people."

The president also took questions for the first time about the investigation into the Boston Marathon bombings that rattled the nation two weeks ago. He defended the FBI's 2011 investigation into Tamerlan Tsarnaev, the suspect who was killed, a probe that resulted in the bureau finding no evidence that he was a threat to the United States.

Russia has since provided more information about Tsarnaev and his mother — both ethnic Chechens— that could have resulted in a more rigorous FBI investigation.

Obama pointedly said that Moscow has been co-operative "since the Boston bombings." He made no reference to information being held back ahead of the attack, but he did say, "Old habits die hard. There are still suspicions sometimes between our intelligence and law enforcement agencies that date back 10, 20, 30 years, back to the Cold War."

Chemical weapons questions

Russia has also stymied U.S. efforts at the United Nations to mount pressure against Assad's embattled government in Syria.

Assad has refused to let a UN team into the areas near Damascus and Aleppo where chemical weapons are believed to have been used. The White House says the team is standing by and could deploy to Syria within 48 hours if Assad allows it in. Given the unlikelihood of Assad giving the inspectors access, the U.S. says it is also seeking answers on its own and through international partners.

Polling suggests war-weary Americans are reluctant to see the U.S. get involved in another conflict in the Middle East. A CBS News/New York Times poll out Tuesday shows 62 percent of Americans say the country does not have a responsibility to intervene in the fighting in Syria, while 24 percent say the government does have that responsibility.

While Obama insists all options are on the table when it comes to dealing with Syria, the White House has little appetite for putting American soldiers into combat there. Even Arizona's Republican Senator John McCain, who has pressed for aggressive U.S. involvement, has said putting U.S. troops on the ground in Syria would be a mistake.

Underscoring the danger that could await, the leader of Lebanon's Hezbollah militant group said Tuesday that Syrian rebels will not be able to defeat Assad's forces by themselves, suggesting the government's friends, including his Iranian-backed group would intervene on the government side if necessary.

Hezbollah and Iran are close allies of Assad, both accused by rebels of sending fighters to assist Syrian troops.