Syrian state television said soldiers found chemical weapons on Saturday in tunnels that had been used by rebels, deflecting blame for a nerve gas attack that killed hundreds this week and heightened Western calls for foreign intervention.
The United States said it was realigning naval forces in the Mediterranean to give President Barack Obama the option of attacking Syria, and a senior U.N. official arrived in Damascus to seek access for inspectors to the gas attack site.
Opposition accounts that between 500 and well over 1,000 civilians were killed by gas fired by pro-government forces, and video footage of victims' bodies, have stoked demands abroad for a robust, U.S.-led response after 2-1/2 years of international inaction on a conflict that has killed 100,000.
International medical humanitarian organization Doctors Without Borders (MSF) said on Saturday that three hospitals it supports in Damascus reported receiving 3,600 people displaying neurotoxic symptoms within less than three hours on Wednesday.
Of those patients, 355 died, and it had sent 7,000 vials of atropine, a nerve agent toxicity antidote, to the area, it said.
In an attempt to strengthen government denials, state news agency SANA said soldiers "suffered from cases of suffocation" when rebels used chemical weapons against them in the Damascus suburb of Jobar.
It said clashes were still raging in the area but that the army had advanced and found "chemical agents" in rebel tunnels.
State television said rebels used poison gas "as a last resort" after government forces made "big gains" in Jobar.
But footage did not appear to show evidence of chemical weapons. It showed five blue and green plastic drums, normally used to transport oil, lined against a wall in a room, as well as several rusty mortar bombs and grenades.
Next to them were several rolls of tape, rope and some gas canisters, normally used for domestic ovens. Gas masks were seen near some vials labeled "atropine".
The presenter said that these images were proof that the rebels had used chemical weapons but did not say which of the items contained them.
Activists say President Bashar al-Assad's forces fired nerve gas projectiles into Jobar and other suburbs before dawn on Wednesday. Later in the week, activists crossed front lines around Damascus to smuggle out tissue samples from victims.
Leader of the opposition Syrian National Coalition Ahmad al-Jarba and the head of the rebel Free Syrian Army General Salim Idriss denied on Saturday that rebels had used chemical weapons.
At a press conference in Istanbul Idriss said the rebels would respond to the attack but would not commit "similar crimes", referring to chemical weapons.
Jabra said the "most important cause" of the attack was the lack of action by the international community, the West in particular, and its silence.
Obama has long been hesitant to intervene in Syria, wary of its position straddling fault lines of wider sectarian conflict in the Middle East, and he reiterated such reluctance on Friday.
But, in a development that could raise pressure on Obama to act, American and European security sources said U.S. and allied intelligence agencies had made a preliminary assessment that chemical weapons were used by pro-Assad forces this week.
Major world powers - including Russia, Assad's main ally which has long blocked U.N.-sponsored intervention against him - have urged the Syrian leader to cooperate with a U.N. inspection team that arrived on Sunday to pursue earlier allegations of chemical weapons assaults in the civil war.
U.N. High Representative for Disarmament Affairs Angela Kane arrived to Damascus on Saturday to press for access to areas of Damascus suburbs said to have been targeted on Wednesday.
"The solution is obvious. There is a United Nations team on the ground, just a few kilometers away. It must very quickly be allowed to go to the site to carry out the necessary tests without hindrance," French Foreign Minister Laurent Fabius said on Saturday during a visit to the Palestinian territories.
Foreign Minister Guido Westerwelle of Germany said it expected Russia to "raise the pressure on Damascus so that the inspectors can independently investigate".
President Hassan Rouhani of Iran, Assad's most powerful Middle East ally, acknowledged on Saturday for the first time chemical weapons had killed people in Syria and called for the international community to prevent their use.
Among the military options under consideration are targeted missile strikes on Syrian units believed responsible for chemical attacks or on Assad's air force and ballistic missile sites, U.S. officials said. Such strikes could be launched from U.S. ships or combat aircraft capable of firing missiles from outside Syrian airspace, thereby avoiding Syrian air defenses.
Obama's caution contrasted with calls for action from NATO allies, including France, Britain and Turkey, where leaders saw little doubt Assad's forces were behind the chemical attack.
While the West accused Assad of a cover-up by preventing the U.N. team from heading out to Damascus suburbs, Russia said the rebels were impeding an inquiry and that Assad would have no interest in using poison gas for fear of foreign intervention.
Igor Morozov, another senior pro-Kremlin lawmaker, told Interfax news agency, "Assad does not look suicidal. He well understands that in this (chemical attack) case, allies would turn away from him and ... opponents would rise. All moral constraints would be discarded regarding outside interference."
Alexei Pushkov, pro-Kremlin chairman of the international affairs committee in Russia's lower house of parliament, said, "In London they are 'convinced' that Assad used chemical weapons, and earlier they were 'convinced' that there were weapons of mass destruction in Iraq. It's the same old story."
Russia said last month that its analysis indicated a projectile that hit the city of Aleppo on March 19 contained the nerve agent sarin and was most likely fired by rebels.