Ammon News - WASHINGTON/BEIRUT | (Reuters) - U.S. President Barack Obama considered options on Saturday for a possible military strike on Syria in response to a nerve gas attack that killed hundreds as Syria sought to avert blame by saying its soldiers had found chemical weapons in rebel tunnels.

A senior U.N. official arrived in Damascus to seek access for inspectors to the site of last Wednesday's attack, in which opposition accounts say between 500 and well over 1,000 civilians were killed by gas fired by pro-government forces.

In the most authoritative account so far, the medical charity Medecins Sans Frontieres (MSF) said three hospitals near Damascus had reported 355 deaths in the space of three hours out of about 3,600 admissions with nerve gas-type symptoms.

The accounts and video footage of the victims - men, women and children - have heightened Western calls for a robust, U.S.-led response after 2-1/2 years of international inaction on a conflict that has killed 100,000 people.

U.S. military and national security advisers met Obama at the White House on Saturday to consider options for a response, the day after Washington said it was realigning forces in the Mediterranean to give him the option of attacking Syria.

Obama, long hesitant to intervene, said in a CNN interview broadcast on Friday that the United States was still gathering information about the attack.

He noted, however, that chemicals weapon use on a large scale would start "getting to some core national interests that the United States has, both in terms of us making sure that weapons of mass destruction are not proliferating, as well as needing to protect our allies, our bases in the region".

In a development that could raise pressure on him to act, American and European security sources said U.S. and allied intelligence agencies had made a preliminary assessment that chemical weapons had been used by pro-Assad forces this week.

MILITARY OPTIONS

Among the military options under consideration are missile strikes on Syrian units believed to be 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 from combat aircraft capable of firing missiles from outside Syrian airspace, thereby avoiding Syrian air defenses.

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 U.N. chemical weapons inspectors already in Damascus to pursue earlier allegations.

Syria accuses rebels of staging the attack to provoke intervention. State television said soldiers had found chemical weapons on Saturday in tunnels that had been used by rebels.

A presenter said five blue and green plastic storage drums shown in video footage, along with rusty mortar bombs, grenades, domestic gas canisters and vials labeled "atropine", a nerve gas antidote, were proof that rebels had used chemical weapons.

Separately, the state news agency SANA said soldiers had "suffered from cases of suffocation" when rebels used poison gas "as a last resort" after government forces made "big gains" 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.

The 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, but not with "similar crimes".

Jabra said the "most important cause" of the attack was the silence and inaction of the international community, especially the West.

A scheduled August 25-27 conference of military chiefs of the United States, Jordan, its main Western allies and Turkey, Saudi Arabia and Qatar, intended to help contain the fallout of a war spilling beyond Syria's borders, has been given added urgency by the gas attack.

WAITING FOR OBAMA

"We had been expecting to talk mainly about stabilizing Jordan," said a European defense source. "Instead, it will be dominated by Syria. It's all really waiting on the Americans and what they decide they want to do ...

"There have been discussions, but so far they have been very inconclusive. As the scale of what happened in Damascus becomes clear, that may change."

U.N. High Representative for Disarmament Affairs Angela Kane arrived in Damascus to press for access to the scene.

"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 during a visit to the Palestinian territories.

German Foreign Minister Guido Westerwelle said Berlin expected Russia to "raise the pressure on Damascus so that the inspectors can independently investigate".

While some of the United States' NATO allies, including France, Britain and Turkey, have explicitly blamed Assad's forces for the chemical attack, Russia said the rebels were impeding an inquiry and that Assad would have no interest in using poison gas for fear of foreign intervention.

"Assad does not look suicidal," senior pro-Kremlin lawmaker Igor Morozov told Interfax news agency. "He well understands that in this 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.

President Hassan Rouhani of Iran, Assad's most powerful Middle Eastern ally, acknowledged for the first time on Saturday that chemical weapons had killed people in Syria and called for the international community to prevent their use.