WASHINGTON Aug 20 (Reuters) - President Barack Obama on Monday warned Syrian President Bashar al-Assad that the use or deployment of chemical or biological weapons in his country's conflict would be a "red line" for the United States as it views Damascus's suppression of the uprising. 
 
 Noting that he had not "at this point" ordered U.S. military engagement in Syria, Obama told reporters during an unscheduled appearance in the White House briefing room that the issue of chemical and biological weapons was of high concern to the both the United States and its close ally, Israel. 
 
 "A red line for us is (if) we see a whole bunch of chemical weapons moving around, or being utilized. That would change my calculus," Obama said. ||||| President Obama defended his campaign's incessant attacks on Mitt Romney over the issue of the GOP nominee's tax returns. 
 
 "If you want to be president of the United States, your life's an open book when it comes to things like your finances," Obama said in a surprise appearance in the White House briefing room. 
 
 "This is pretty standard stuff," Obama said. "I don't think we're being mean." 
 
 "It's what the American people expect," he added. 
 
 Romney has so far refused to release any more than one year's worth of tax returns — in contrast to long-standing disclosure traditions dating back to Romney's own father in 1968. 
 
 "I think the idea that this is somehow exceptional, that there should be a rationale or a justification for doing more than the very bare minimum has it backwards," Obama said. "The assumption should be you do what previous presidential candidates have done dating back decades." 
 
 Obama also denied that his campaign has charged that Romney is doing something illegal — in contrast to some of the rhetoric of his campaign aides. 
 
 "There's a difference between playing by the same sets of rules and doing something illegal. In no way have we suggested the latter," Obama said. 
 
 That statement runs contrary to what Obama's own deputy campaign manager Stephanie Cutter said in July, when she told reporters that Romney either lied to the American people or committed a felony. 
 
 “Either Mitt Romney, through his own words and his own signature, was misrepresenting his position at Bain to the SEC, which is a felony," Cutter said on a conference call.
– President Obama gave perhaps the strongest indication yet today that the US is prepared to use military force in Syria. In a surprise appearance in the White House briefing room, Obama said he has not ordered military action "at this point," but that he hadn't ruled it out, Reuters reports. "A red line for us is (if) we see a whole bunch of chemical weapons moving around, or being utilized," he said. "That would change my calculus." In the same appearance, Obama defended his campaign's attacks on Mitt Romney's refusal to release his tax returns. "If you want to be president of the United States, your life's an open book when it comes to things like your finances," he said, according to Politico. "This isn't overly personal here, guys. This is pretty standard stuff. I don't think we're being mean by asking you to do what every other presidential candidate has done!"