Washington (CNN) -- While Syrian rebel forces have made significant military advances on the ground against the forces of President Bashar al-Assad in recent days, the U.S. ambassador to Syria says there's no imminent end to the fighting.
"It's very clear to me that the regime's forces are being ground down," Robert Ford said Thursday at a conference sponsored by the Foundation for the Defense of Democracies in Washington. "That said, the regime's protection units continue to maintain some cohesion, and they still have some fight left in them, even though they are losing. I expect there will be substantial fighting in the days ahead."
The fighting has taken a more severe turn in the last week, with U.S. officials now concerned the Syria's president could use chemical weapons out of desperation. This intensifying 20-month conflict has frustrated those who have long argued that the U.S. should intervene militarily.
"The longer this conflict has gone, the worse it has gotten," Sen. John McCain (R-Arizona) said at a news conference Thursday alongside fellow Sens. Joseph Lieberman (I-Connecticut) and Lindsay Graham (R-South Carolina). "All of those who argued for non-intervention because of the things that might happen have now happened because we failed to intervene. And the fact is that we have now reached a point where there are weapons of mass destruction that may be used, and also there is a significant question about the security of these weapons should Bashar Assad fall."
With no end to the fighting in sight, the United States and other western groups fear that extremist groups already operating on the ground with rebel forces will exploit the instability to further their own agendas.
As CNN reported earlier this week, the State Department is planning to designate the al-Nusra Front, a radical Islamist group in Syria, as a foreign terrorist organization as Secretary of State Hillary Clinton prepares to lead the U.S. delegation at a conference on Syria in Morocco next week. Efforts are also under way for the United States to formally recognize the newly formed Syrian political opposition group as the legitimate representative of the Syrian people, which France and Britain have already done.
Both the terror designation and the assurance that a political opposition represents the range of ethnicities and religions inside Syria are necessary, officials say, to lessen the possibility of Syria disintegrating into ethnic bloodshed. Such an outcome would pit the majority Sunni population against Assad's minority Alawite sect if Assad were to leave.
"The longer the violence goes on, the more extremist groups like al-Nusra benefit," Ford said. "Based on what we have seen of al-Nusra's parent organization, al Qaida in Iraq, al-Nusra will not be merciful at all to the Alawites. And so it is incumbent on us to bolster moderates (in the opposition), which is what we are trying to do.
As fears over the possible use of chemical weapons by the Assad regime loom, Ford said the United States' posture toward the Syrian civil war would shift in a "fundamental way" if the regime used them.
"What must be understood is that the use of chemical weapons is for us a qualitatively different situation," Ford said. "It will change our calculation in a fundamental way, it will change the way we have approached the Syrian problem. Let me leave it at that."
The ambassador spoke as the pressure to find a diplomatic solution to the crisis intensifies. In Dublin, Ireland, Clinton met with Russian Foreign Minister Sergey Lavrov on the sidelines of a conference on European security. She held a separate meeting with Lavrov and Lakhdar Brahimi, the United Nations special envoy to Syria.
While the United States has expressed its frustration with Russian and Chinese vetoes on resolutions regarding Syria at the United Nations Security Council, it was unclear whether the meetings might signal a shift in Russia's approach to Syria.
"We have been trying hard to work with Russia to stop the bloodshed in Syria and start a political transition toward a post-Assad Syrian future," Clinton told reporters at a news conference with Irish Prime Minister Enda Kenny. "And we very much support what Lahkdar Brahimi is trying to do."
A senior State Department official said Clinton, Lavrov and Brahimi met for about 40 minutes, followed by a short follow-up meeting between Clinton and Brahimi.
"It was a constructive discussion focused on how to support a political transition in practical terms," the official said.
But the question of what should happen to Assad if he stays in Syria is for the Syrian people to decide.
"If in the end the Syrians decide it is better not to pursue Bashar al-Assad, that is a Syrian decision," Ford said. "But we do think that the extent of the brutality and the extent of the violence require at least some level of accountability."