The White House has strong indications the Assad regime used nerve gas in attacks Syrian rebels claim killed more than 1,100 people,  a U.S. official said.
The Security Council, in an emergency meeting in New York, called for a prompt investigation of the allegations, which came two days after a U.N. team began looking into earlier claims of chemical-weapons use by both sides.
At least five towns were gassed in the pre-dawn attacks in Damascus suburbs where rebels have had recent success in fighting off regime forces, opposition groups and activists said.
Witnesses said Russian-type Grad rockets -- similar to those the Palestinian Sunni Islamic group Hamas has fired into Israel -- started falling about 2 a.m.
Dozens of videos posted online showed scores of lifeless bodies sprawled on floors. Some men were wrapped in burial shrouds, some children were in diapers. No wounds were visible.
Other videos showed children on makeshift hospital floors vomiting, convulsing and struggling to breathe. Some were being treated with hand-held respirators while others received cardiopulmonary resuscitation.
Men sprawled on the floor were shown being hosed down.
Women were killed and treated too, but they were not recorded on video out of respect, opposition activists said.
The Western-backed Syrian Opposition Coalition  said more than 1,100 people died.
The Local Coordination Committees of Syria, a network of opposition activists, said at least 1,360 were killed.
The United Medical Office of al-Ghouta, an umbrella group for opposition-run field hospitals, put the number of dead at 1,600.
The alleged attack sites weren't accessible to journalists, so it was not immediately possible to confirm the allegations or casualty counts.
The indications are preliminary and we do need to do our due diligence and get all the facts and determine what steps need to be taken, the official said.
But the White House conclusion in June forces loyal to President Bashar Assad used chemical weapons on a small scale a number of times in the past year did not bring about a noticeable shift in U.S. engagement.
White House spokesman Josh Earnest said Wednesday the administration was deeply concerned about the reports.
The United States, the European Union and other world powers called for the 13 U.N. weapons inspectors in Syria investigating other sites of alleged chemical-weapons attacks to immediately visit Wednesday's attack sites.
The team is based 5 to 10 miles away.
Russia, a regime supporter, suggested rebels had launched a chemical attack and blamed it on the regime to win U.N. support and thwart a planned peace conference in Geneva, Switzerland.
"All of this really looks like an attempt, at any cost, to create a reason to produce demands for the U.N. Security Council to side with the regime's opponents and undermine the chances of convening the Geneva conference", Foreign Ministry spokesman Alexander Lukashevich said in a statement.
The Security Council, meeting in emergency session, called for a prompt investigation of the allegations and appealed for a cease-fire, but took no further action.
"I can say that there is a strong concern among council members about the allegations and a general sense that there must be clarity on what happened, and that the situation has to be followed carefully", Security Council President Maria Perceval of Argentina said after the closed-door meeting.
All council members agreed that any use of chemical weapons, by any side under any circumstances, is a violation of international law, she said.
British Deputy U.N. Ambassador Philip Parham said on Twitter late Wednesday night 37 countries had signed a letter to Secretary-General Ban Ki-moon formally requesting Syrian authorities grant the U.N. investigators "urgent access to all relevant sites and information sources.
"All, including Russia, welcomed the secretary-general's determination to ensure a prompt, thorough, impartial investigation", he said on Twitter.
Parham said Moscow's support of Ban's determination to investigate promptly means Russia is telling Syria to agree quickly.