The top U.S. military commander in Europe said Tuesday that several NATO countries are working on contingency plans for possible military action to end the two-year civil war in Syria as President Bashar Assad's regime accused U.S.-backed Syrian rebels of using chemical weapons.

The Obama administration rejected the Assad claim as a sign of desperation by a besieged government intent on drawing attention from its war atrocities -- some 70,000 dead, more than 1 million refugees and 2.5 million people internally displaced.

A U.S. official said there was no evidence that either Assad forces or the opposition had used chemical weapons in an attack in northern Syria.

The chairman of the House Intelligence Committee, Rep. Mike Rogers, R-Mich., said there was a "high probability" that the Assad government had used chemical weapons, although it was not clear whether he was referring to the attack in northern Syria. "We need that final verification, but given everything we know over the last year and a half, I would come to the conclusion that they are either positioned for use, and ready to do that, or in fact have been used," Rogers told CNN.

As the war enters its third year, the U.S. military, State Department officials and the U.N. high commissioner for refugees delivered a dire assessment of a deteriorating situation in Syria and the sober view that even if Assad leaves, the Middle East nation could slip into civil strife and ethnic cleansing similar to the Balkans in the 1990s.

"The Syrian situation continues to become worse and worse and worse," Adm. James Stavridis, the commander of U.S. European Command, told the Senate Armed Services Committee. "No end in sight to a vicious civil war."

Stavridis, who is retiring soon, said a number of NATO nations are looking at a variety of military operations to end the deadlock and assist the opposition forces, including using aircraft to impose a no-fly zone, providing military assistance to the rebels and imposing arms embargoes.

As with U.S. and international involvement in Libya in 2011, a resolution from the U.N. Security Council and agreement among the alliance's 28 members would be necessary before NATO assumes a military role in Syria, Stavridis said.

"We are prepared if called upon to be engaged as we were in Libya," he said.

But within individual member countries, the admiral said, "there's a great deal of discussion" about lethal support to Syria, no-fly zones, arms embargoes and more. "It is moving individually within the nations, but it has not yet come into NATO as an overall NATO-type approach," he said.

Senate Armed Services Chairman Carl Levin, D-Mich., asked whether there is any consideration of targeting Syria's air defenses. Stavridis simply said yes.

NATO has installed Patriot missile defense batteries in southern Turkey along the border with Syria that are also capable of shooting down aircraft. During an exchange with Sen. John McCain, R-Ariz., Stavridis said the Patriots could be positioned in such a way as to shoot down Syrian aircraft, and he indicated that doing so would be a powerful disincentive for pilots to fly in that area.

Turkey's leaders have been "very emphatic" that the missiles be used only for defensive purposes, Stavridis said. To use the batteries for other missions, including attacking Syrian military aircraft, would require consensus among NATO's members -- "and we're far from that," Stavridis told the committee.

Stavridis said that his personal opinion is that providing military assistance to the Syrian opposition "would be helpful in breaking the deadlock and bringing down the Assad regime."

Syria's state-run news agency said 25 people were killed in a chemical attack on the Khan al-Assad village in northern Aleppo province. It said 86 people were wounded, some critically, and published pictures of children and others on stretchers in what appeared to be a hospital ward.

Russia, which has steadfastly supported Assad in Syria's civil war, on Tuesday backed Assad's assertion of a chemical attack.

White House spokesman Jay Carney said the U.S. is looking carefully at all allegations but that the Obama administration is "deeply skeptical" of any claims emanating from Assad's regime.

Denis McDonough, the White House chief of staff, said the U.S. wouldn't stand by if it turns out the regime used chemical weapons, but he declined to say whether he believed the reports could be true.

"If this is substantiated, it does suggest ... that this is a game-changer. And we will act accordingly," McDonough told CNN. "This is something we take very, very seriously."

Syria has one of the world's largest arsenals of chemical weapons and Washington has been on high alert since last year for any possible use or transfer of chemical weapons by Assad's forces. It feared that an increasingly desperate regime might turn to the stockpiles in a bid to defeat the rebellion or transfer dangerous agents to militant groups such as Lebanon's Hezbollah, which the Syrian government has long supported.

At the time, officials noted movement of some of the Syrian stockpiles but said none appeared to be deployed for imminent use. Still, President Barack Obama declared the use, deployment or transfer of the weapons to be his "red line" for possible military intervention in the Arab country.

Sen. Lindsey Graham, R-S.C., a member of the Armed Services Committee, raised the prospect of deploying U.S. troops to Syria to secure the stockpile of chemical weapons.

"If the choice is to send in troops to secure the weapons sites versus allowing chemical weapons to get into the hands of some of the most violent people in the world, I vote to cut this off before it's a problem," Graham told a group of reporters. "This administration's handling of Syria is going to cause incredible problems in the Mideast and compromise our national security."

But another member of the committee, Sen. Kelly Ayotte, R-N.H., said the United States "should take every step that we can short of boots on the ground."

At a separate congressional hearing, AntDonio Guterres, the U.N. high commissioner for refugees, said the international community faces a "tipping point" in Syria, with a fast-moving refugee crisis.

The number of refugees arriving in neighboring countries has jumped to 14,000 in a 24-hour period, up from 3,000 in December, 5,000 in January and 8,000 in February, Guterres said. Lebanon has 360,000 registered Syrians, Jordan more than 350,000 and Turkey some 260,000.

"The refugee crisis has been accelerating since last summer, and has reached staggering proportions since the beginning of this year," Guterres told a Senate Foreign Relations subcommittee. He said the world community needs "to be prepared for things to get worse before they get better."

After Assad falls, Syria could end up like the Balkans, Stavridis told the Armed Services panel.

"We saw in the Balkans 100,000 killed, 1 million people, 2 million people pushed across borders, (and) two significant wars, one in Bosnia, Herzegovina, one in Serbia, Kosovo," Stavridis said. "I think, unfortunately, that's probably the future in Syria. It's going to be after the Assad regime falls. I think, there's going to be every potential for a great deal of revenge killing, interreligious conflict between various segments of the population, and it's very difficult to see the pieces of Syria going back together again very easily."

The violent, unending war has prompted some in Congress to offer legislation and demand greater action by the Obama administration. But a war-weary American public has been slow to embrace many of the efforts.

In the latest proposal, Sens. Bob Casey, D-Pa., and Marco Rubio, R-Fla., offered a bipartisan measure that would provide non-lethal aid to vetted Syrian opposition groups battling the Assad regime, such as body armor and communications equipment.

Casey and Rubio left open the possibility of arming the rebels at a later date.

"Down the road we may make another determination," Casey said when asked about arming the rebels.