President Barack Obama declared Syria's main opposition group the sole “legitimate representative” of its country's people Tuesday, deeming the move “a big step” in the international diplomatic efforts to end Syrian President Bashar Assad's embattled regime.

Obama said the newly formed Syrian Opposition Council “is now inclusive enough” to be granted the elevated status, which paves the way for the greater U.S. support for the organization.

“Obviously, with that recognition comes responsibilities,” Obama said in an interview Tuesday with ABC News. “To make sure that they organize themselves effectively, that they are representative of all the parties, that they commit themselves to a political transition that respects women's rights and minority rights.”

Recognition of the council as the sole representative of Syria's diverse population brings the U.S. in line with Britain, France and several of America's Arab allies, which took the same step shortly after the body was created at a meeting of opposition representatives in Qatar last month.

Obama's announcement follows his administration's blacklisting of a militant Syrian rebel group with links to al-Qaida. That step is aimed at blunting the influence of extremists amid fears that the regime may use or lose control of its stockpile of chemical weapons.

Defense Secretary Leon Panetta said Tuesday that the Syrian government seems to have slowed preparations for the possible use of chemical weapons against rebel forces. Last week, U.S. officials said there was evidence that Syrian forces had begun preparing sarin, a nerve agent, for possible use in bombs.

“At this point the intelligence has really kind of leveled off,” Panetta told reporters traveling with him to Kuwait, where he will visit U.S. troops at the start of a four-day trip. “We haven't seen anything new indicating any aggressive steps to move forward in that way.”

U.S. recognition of the opposition council is expected to be a centerpiece of an international conference on the Syria crisis in Morocco this week. Secretary of State Hillary Rodham Clinton had been due to attend Wednesday's meeting in Marrakech but canceled her trip because she was ill with a stomach virus, her spokesman, Philippe Reines, said. Instead, Deputy Secretary of State William Burns will lead the U.S. delegation.

On Monday, Clinton designated Jabhat al-Nusra, or “the Support Front” in Arabic, a foreign terrorist organization. The move freezes any assets its members may have in U.S. jurisdictions and bars Americans from providing the group with material support. The designation is largely symbolic because the group is not thought to have holdings or support in the United States, but officials hope the penalties will encourage others to take similar action and discourage Syrians from joining.

That step was part of a package intended to help the leadership of the Syrian Opposition Council improve its standing and credibility as it pushes ahead with planning for a post-Assad future.

The administration took further action Tuesday against extremists on both sides, with the Treasury Department setting separate sanctions against two senior al-Nusra leaders and two militant groups operating under the control of the Syrian government. Two commanders of the pro-Assad shabiha force also were targeted.

“We will target the pro-Assad militias just as we will the terrorists who falsely cloak themselves in the flag of the legitimate opposition,” said David S. Cohen, the department's sanctions chief.

More significant, though, is the upgraded status for the council. It's expected to be accompanied by pledges of additional humanitarian and nonlethal logistical support for the opposition. It's unlikely that the U.S. would add military assistance to that, at least in the short-term. Providing arms remains a matter of intense internal debate inside the administration, officials said.

The U.S. had been leading international efforts to prod the fractured Syrian opposition into coalescing around a leadership that would truly represent all of the country's factions and religions. Yet it had held back from granting recognition to the group until it demonstrated that it could organize itself in credible fashion.

In particular, Washington had wanted to see the group set up smaller committees that could deal with specific immediate and short-term issues, such as governing currently liberated parts of Syria and putting in place institutions to address the needs of people once Assad is ousted. Some of those committees could form the basis of a transitional government.

Officials said the U.S. evolution in recognizing Syria's opposition would closely mirror the process the administration took last year in Libya with its opposition.

“I would remind you of how this went in the Libya context where we were able to take progressive steps as the Libyan opposition themselves took steps to work with them, and to advance the way we dealt with them politically,” State Department spokeswoman Victoria Nuland said Monday.

In that case, Libya's National Transitional Council moved from being “a” legitimate representative to “the” legitimate representative of the Libyan people. While the revolution was still going on, the council then opened an office in Washington, and the administration sent the late Ambassador Chris Stevens to Benghazi, Libya, as an envoy in return. The move also opened the door for Libya's new leaders to access billions of dollars in assets frozen in U.S. banks that had belonged to the Gadhafi regime.

The move could allow the Syrian opposition to set up a liaison office in Washington with a de facto ambassador.

It is unclear, however, given the level of violence in Syria and the potential threat of chemical weapons, if the U.S. would soon send a representative to rebel-controlled areas of the country.

The conflict started 20 months ago as an uprising against Assad, whose family has ruled the country for four decades. It quickly morphed into a civil war, with rebels taking up arms to fight back against a bloody crackdown by the government. According to activists, at least 40,000 people have been killed since March 2011. (AP)