Syrian opposition says it is fully committed to getting U.N. inspectors to site of attack President Barack Obama says signs point to a "big event of grave concern" Russia suggests Syrian opposition is blocking access to the alleged attack site Damascus (CNN) -- U.N. inspectors in Syria face a race against time to get to the site of an alleged chemical weapons attack to gather vital evidence, but the big question Friday was whether red tape would prevent that. Since Friday was not a working day in Syria, government offices were closed and many government officials were off. The Syrian government has denied using chemical weapons. Syrian rebels promise access Russia's Foreign Ministry appeared Friday to accuse the Syrian opposition of blocking U.N. access to the site. The ministry earlier Friday said that Foreign Minister Sergey Lavrov had called for both the Syrian government and opposition to allow access, in a conversation with U.S. Secretary of State John Kerry. On Wednesday, the ministry had seemed to imply that the opposition was responsible for the use of chemical weapons, issuing a statement saying that an "improvised missile was launched from the positions occupied by militants" and that an "aggressive information attack" was designed to fool the world into believing that the Syrian regime was responsible. On Friday, leaders of the Syrian National Coalition, an umbrella opposition group, told reporters in Istanbul that the opposition and leaders of the Free Syrian Army are "fully committed" to helping the U.N. inspectors reach all locations where chemical weapons were allegedly used. "We hereby assure the U.N. inspectors, the U.N. secretary general and all members of the United Nations that we will ensure the safety of the U.N. inspectors' team," said spokesman Khaled al-Saleh. However, it is very critical to get that team into the area that was just hit in less than 48 hours. The clock is ticking, we want to see those inspectors and we believe that the evidence will show who used these chemical weapons against innocent civilians. Al-Saleh said that medical teams in the area hit administered 25,000 shots of atropine -- a medication used to treat people exposed to the nerve gas sarin -- after the attack, and that the toll would have been much higher if that had not been done. Syrian National Coalition Secretary General Badr Jamous said that samples had been obtained from the area -- which he said had been besieged by government forces for months -- and were being sent outside Syria for analysis. Jamous detailed the firing of rockets -- some with chemical warheads and others conventional weapons -- early in the morning into what he said was a heavily populated civilian area. More than 1,300 were killed, most of them as a result of the use of chemical weapons against civilians, said Saleh -- all while the U.N. inspection team was only 10 kilometers (6 miles) away. Jamous said the attack also injured more than 5,000 people. The chemical component used has not yet been identified, Jamous said, but the number of casualties involved means the U.N. team should be moving in urgently to check the scene. Obama: 'Big event of grave concern' U.S. military officials have updated options for a forceful intervention in Syria, a senior defense official tells CNN. The Joint Staff and U.S. Central Command last conducted a major update of options in April, in response to bipartisan pressure from congressional Republicans and Democrats. Target lists for possible airstrikes were updated, including government buildings and military installations, but Syrian President Bashar al-Assad's forces and equipment "continue to move," and thus require flexibility in planning, the official said. The planning, meant to give Obama "a current and comprehensive range of choices," also included updates on the potential use of cruise missiles, which would not require U.S. pilots to enter Syrian airspace, the official said. No decision was made at a national security meeting Thursday at the White House, and no formal determination has been made on whether the Syrian government used chemical weapons, the official said. On Friday afternoon, reporters asked White House deputy press secretary Josh Earnest about Syria. We've said the assistance we provide to the opposition is on an upwards trajectory, expanding in scope and in scale. We have long said that all options remain on the table when it comes to Syria. The president has indicated clearly that he does not foresee a situation in which American boots on the ground would be in the best interest of U.S. national security. But ultimately, that is the criteria he will use when he evaluates the proper course of action in this situation. When reports of a possible chemical attack came in on Wednesday morning, U.S. Ambassador Samantha Power called for immediate consultations with the Security Council. The U.S. mission sent a joint letter to Secretary-General Ban, along with 36 other countries, that evening asking for an immediate investigation, a source with the mission told CNN. Power was out of the office on personal travel at that time, but returned to her New York office on Friday, the source said. The images of victims of the alleged attack, including many children, are "heartbreaking and sickening," Ban has said. Ban said the situation in Syria, where rebels have been fighting al-Assad's forces for more than two years, continues to worsen. Two U.N. agencies announced Friday that the number of child refugees from Syria has passed a landmark threshold, with 1 million forced to flee during the conflict. An al-Assad government spokesman said any claims it used chemical weapons are "illogical and fabricated." Syrian Information Minister Omran Zoabi told state TV the claim was timed by the opposition to coincide with the U.N team's visit and came as government forces were making gains on all sides against the rebels. Obama has directed the U.S. intelligence community to urgently gather additional information to try to assess whether chemical weapons were used Wednesday, State Department spokeswoman Jen Psaki said Thursday. Meanwhile, a U.S. defense official said Friday the United States has added one more destroyer to the eastern Mediterranean fleet. The USS Ramage arrived to replace the USS Mahan, but the Mahan is going to stay around a bit longer, so temporarily there will be four in the fleeet. The USS Gravelly and USS Barry are still there. CNN's Frederik Pleitgen reported from Damascus and Laura Smith-Spark wrote and reported in London. CNN's Chris Lawrence, Samira Said, Arkady Irshenko, Alla Eshchenko, Richard Allen Greene and Becky Anderson contributed to this report.