BEIRUT (AP) -- Syrian troops and opposition fighters clashed Friday during fierce battles in suburbs of the Syrian capital where the opposition claimed a chemical weapons attack this week killed more than 100 people, activists said. Also Friday, the Lebanese government said its troops captured a truck carrying gas masks near the Syrian border. The government offensive entered its third day and came as U.N. Secretary-General Ban Ki-moon urged the Syrian government to allow a U.N. team now in Damascus to swiftly investigate the alleged chemical weapons attack. U.N. deputy spokesman Eduardo del Buey said the U.N. chief has been in touch with world leaders since Wednesday and is sending U.N. disarmament chief Angela Kane to Damascus to press for an investigation. Syrian opposition figures and activists have reported death tolls from Wednesday's attack ranging from 136 to 1,300. But if confirmed, even the most conservative tally would make it the deadliest chemical attack in Syria's civil war. In an interview broadcast Friday on CNN, U.S. President Barack Obama called the possible chemical attack in Syria a "big event of grave concern" and said, while pending confirmation, the event was "very troublesome" and was going to "require America's attention." A 20-member U.N. team led by Swedish chemical weapons expert Ake Sellstrom has been in Damascus since Sunday to investigate three sites where past chemical weapons attacks allegedly occurred: the village of Khan al-Assal just west of the embattled northern city of Aleppo and two other locations kept secret for security reasons. Deputy Prime Minister Qadri Jamil told The Associated Press Thursday he was personally in favor of a fair, transparent international delegation to investigate the most recent incident in Ghouta. But he said that would require a new agreement between the government and the U.N. and that the conditions for such a delegation would need to be studied. In neighboring Lebanon, the army said in a statement that troops have captured a truck with "large amounts" of gas masks in the southeastern village of Kfeir near the border with Syria. A Lebanese army general said an investigation is under way about the masks and could not say whether they were being taken to Syria. He spoke on condition of anonymity in line with regulations. The U.K.-based Syrian Observatory for Human Rights and Syria-based activist Mohammed Abdullah said the fighting, air raids and shelling occurred in different parts of Damascus suburbs Friday. "Clashes are almost on all fronts," said Abdullah, who is based in the Damascus suburb of Saqba. "Zamalka is destroyed," he said referring to one of the areas that was allegedly struck with chemical weapons on Wednesday. In Moscow, the Russian Foreign Ministry called for an independent probe by U.N. experts into the alleged attack. The statement released on Friday said that Minister Sergey Lavrov and U.S. Secretary of State John Kerry had discussed the situation by telephone on Thursday, and concluded that they had a "mutual interest" in calling for the U.N. investigation. The statement said Russia had called for Syrian President Bashar Assad's embattled government to cooperate with an investigation, but questions remained about the willingness of the opposition, "which must secure safe access of the mission to the location of the incident." Russia has been one of Assad's key allies in the international arena. Unrest in Syria began in March 2011 and later exploded into a civil war. More than 100,000 people have been killed in the conflict.