Obama Weighing 'Limited' Response on Syria President Obama said today he has not made a decision on whether to launch a retaliatory strike against the Syrian government, but emphasized that when countries "break international norms" by using chemical weapons they must be held accountable. "I have no interest in any kind of open-ended conflict in Syria, but we do have to make sure that when countries break international norms on weapons, like chemical weapons that could threaten us, that they are held accountable," Obama said in an interview today with PBS NewsHour. As lawmakers increasingly call for the president to seek congressional approval for any military action, Obama argued the U.S. must protect its core self-interests. Obama said the U.S. government believes the regime of Bashar al-Assad was responsible for an attack that killed hundreds of people in Syria last week and that the officials believe the opposition does not possess chemical weapons that could be used on that scale. "We want the Assad regime to understand that by using chemical weapons on a large scale against your own people, against women, against infants, against children, that you are not only breaking international norms and standards of decency, but you're also creating a situation where U.S. national interests are affected -- and that needs to stop," he said. The president warned about the broader repercussions of the use of chemical weapons in Syria. "When you start talking about chemical weapons in a country that has the largest stockpile of chemical weapons in the world, where over time their control over chemical weapons may erode, where they're allied to known terrorist organizations that in the past have targeted the United States, then there is a prospect, a possibility in which chemical weapons that can have devastating effects could be directed at us," he said. We want to make sure that that does not happen. The U.N.'s special envoy to Syria announced today that evidence suggests a "chemical substance" was used to kill hundreds of people in Syria last week, but the U.N. pleaded for more time before the U.S. and allies launch a retaliatory strike against the Syrian regime. The announcement came as the U.S., France and Britain appeared to gear up for a military strike on the Syrian regime. Speaking to reporters in Geneva, special envoy Lakhdar Brahimi said, "With what has happened on the 21st of August last week, it does seem that some kind of substance was used that killed a lot of people: hundreds, definitely more than a hundred, some people say 300, some people say 600, maybe 1,000, maybe more than 1,000 people." He added, "International law says that any U.S.-led military action must be taken after" agreement in the U.N. Security Council. U.N. Secretary-General Ban Ki-moon has asked for another couple of days, saying, "the team needs time to do its job" on the ground. The U.N. convoy came under fire on Monday and the team postponed its field visit Tuesday because of security concerns. The council must find the unity to act. Later today, Britain was expected to introduce a resolution to the U.N. Security Council "authorizing necessary measures to protect civilians" in Syria. According to a Downing Street spokesman, the U.K. would call for authorization of all measures under Chapter 7 of the U.N. Charter, which could include breaking diplomatic ties and economic sanctions, or action by air, sea or land forces. British Prime Minister David Cameron tweeted this morning, "we've always said we want the U.N. Security Council to live up to its responsibilities on Syria. A Syrian army source told the Iranian news agency FARS, "If Damascus comes under attack, Tel Aviv will be targeted, too." Iran's Supreme Leader Ayatollah Ali Khamenei also voiced his opposition to a U.S. military strike, calling it a "disaster." Israel's cabinet has called up a limited enlistment of reserve forces and Israeli Army Radio reports Israel has reinforced its missile defenses as a precaution against possible retaliatory attacks. Israeli Prime Minister Netanyahu told his nation today, "There is no reason to change daily routines," and echoed his statement from Tuesday saying, "We are prepared for any scenario." Israeli media reported a run on gas masks and local TV stations showed video of long lines at a free gas mask distribution center in Tel Aviv. ABC News' Alyssa Newcomb and Mary Bruce contributed to this report.