Barack Obama says Syria attacks 'of grave concern' as Russia calls on Assad to allow inspectors He added that there was now "a more abbreviated time frame" for dealing with both Egypt and Syria. Russia, one of the few remaining allies of Assad, has meanwhile told Syria to cooperate with UN experts investigating reports of a deadly chemical weapons attack. Sergei Lavrov, the foreign minister, told his US counterpart that the "Russian side called on the Syrian government to cooperate with the UN chemical experts," the foreign ministry said in a statement. Mr Lavrov called John Kerry, the secretary of state, immediately after the reports first emerged of the attack on Wednesday. "It is now up to the opposition to ensure safe access for the mission to the site of the alleged incident," the statement said. Russia's foreign ministry said both Mr Lavrov and Mr Kerry agreed on the need for an "objective investigation" into what happened. "Common interest was expressed in an objective investigation by a UN expert mission, which is currently in the country, in connection with the reports about the possible use of chemical weapons in a Damascus suburb," the statement said. The Syrian opposition said this week that the Damascus regime used chemical weapons near the capital in an attack that killed hundreds of people. Harrowing footage distributed by activists showing unconscious children, people foaming around the mouth and doctors apparently giving them oxygen to help them breathe has triggered revulsion around the world. The opposition National Coalition said that more than 1,300 people were killed. Syrian activists say they are smuggling out body tissue samples from victims of the attack, and are trying to get them to a team of UN inspectors staying in a hotel a few miles away. "The UN team spoke with us and since then we prepared samples of hair, skin and blood and smuggled them back into Damascus with trusted couriers," said activist Abu Nidal, speaking from the town of Arbin. Only a few activists said they were confident that they had a contact who would be able to hand their samples to the UN inspectors. Several activists in the area who spoke to Reuters said they had prepared samples to smuggle into the capital but were unable to find a way to access the monitors inside their hotel. We're being shelled, and on top of that Ghouta is surrounded by regime checkpoints. But even that isn't a problem, we can smuggle them out," said activist Abo Mohammed, from the suburb of Harasta. The problem is the location of the UN committee in the hotel. They're under heavy guard and government minders. The United Nations formally asked Syria to authorise UN experts currently on a mission in Syria to probe the allegations. Russia, which supported the Assad regime throughout the conflict, said on Wednesday the reports were a "premeditated provocation" aimed at undermining the chances of all sides agreeing to sit down for peace negotiations first mooted by Moscow and Washington in May.