The top U.S. military official warned Tuesday that the uprising in Syria is far more complicated than last year's revolution in Libya, describing it as a regional battle between Shia and Sunni Muslims that involves "all the players" in the Middle East.

Explaining what may be the Obama administration's reticence to take any military action in Syria, Joint Chiefs of Staff Chairman Gen. Martin Dempsey cited a litany of concerns during a hearing before the Senate Armed Services Committee.

Dempsey said the United States does not have "as clean an understanding of the nature of the opposition," though is working to develop that. Plus, he said, Syria poses a "chemical and biological warfare threat" and has a "very credible military."

"That it is a much different situation than we collectively saw in Libya," Dempsey said.

More broadly, the Joint Chiefs chairman described Syria as a battleground between the region's historically opposed Shia and Sunni Muslim populations.

"There's also huge regional implications, big players and actors who have vested interests there," Dempsey said.

He urged officials to look at the conflict in a "regional context," casting the violence in Syria as the last tumultuous phase of the so-called Arab Spring.

"All of the players in the region, it seems, have a stake in this," Dempsey said.

He later added: "And so those who would like to foment a Sunni-Shia standoff, and you know who they are, are all weighing in in Syria. It is the last remaining piece in the puzzle of what you and I probably months ago would have described as the Arab Spring. But this is a very important moment in the region, and all the players are weighing in."

The conflict, which has become increasingly violent and claimed thousands of Syrian lives since last year, has put the United States and its allies in a difficult position. By contrast, Libya was geographically isolated from the Middle East and its late leader Muammar Qaddafi was not as influential in the region as Syria's Bashar Assad. The Assad regime, while being condemned by members of the Arab League, has close ties with Iran and allows the country to be a staging ground for the Hezbollah terror group that Iran funds.

The Obama administration has preferred to deal with Syria via sanctions and international pressure. But the U.S. and its allies recently were unable to push an Arab League-backed resolution at the U.N. Security Council calling for Assad to step down. Russia and China vetoed the proposal.

On Tuesday, China's vice president, Xi Jinping, visited President Obama and Vice President Joe Biden. Administration officials said Syria was discussed but did not offer any details.

Meanwhile, the U.S. has announced an effort to work outside the U.N. system. Secretary of State Hillary Clinton, acknowledging the limitations, said Monday that any plan to send peacekeepers into Syria would take Syria's consent.

Dempsey said Tuesday that the Syrian opposition is "for the most part domestic" despite the influence of "regional actors."

Asked about a report that al Qaeda is calling on Muslims to align with the Syrian opposition, Dempsey said he has "no confirmation" that al Qaeda is actually involved in the country but he also would not discount the possibility.

White House Press Secretary Jay Carney said Tuesday that the administration, along with the group of countries dubbed the "friends of Syria," would continue to pressure Assad through sanctions and move toward providing humanitarian aid to the Syrian people.

Despite the apparent barriers to military involvement, Carney expressed hope that international pressure would compel Assad to cease the "reprehensible violence against his own people."