* Ban: Attack on Syria could make situation there worse
* Security Council must overcome its deadlock
* Syria, Russia say rebels were behind Aug. 21 attack
U.N. Secretary-General Ban Ki-moon said on Tuesday that the use of force is only legal when it is in self-defense or with Security Council authorization, remarks that appear to question the legality of U.S. plans to strike Syria without U.N. backing
He also suggested that a U.S. attack could lead to further turmoil in conflict-ravaged Syria, where the United Nations says over 100,000 people have been killed in the country's 2-1/2-year civil war.
Ban was speaking to reporters after President Barack Obama won the backing of two top Republicans in Congress in his call for limited U.S. strikes on Syria to punish President Bashar al-Assad for his suspected use of chemical weapons against civilians.
"The use of force is lawful only when in exercise of self-defense in accordance with Article 51 of the United Nations charter and/or when the Security Council approves of such action," Ban said. "That is a firm principle of the United Nations."
Obama said on Saturday he was "comfortable going forward without the approval of a United Nations Security Council that so far has been completely paralyzed and unwilling to hold Assad accountable."
Russia, backed by China, has used its veto power in the Security Council three times to block resolutions condemning Assad's government and threatening it with sanctions. Assad's government, like Russia, blames the rebels for the Aug. 21 attack.
The United States has bypassed the United Nations in the past when the council was deadlocked, such as during the Kosovo war in 1999. At that time, Washington relied on NATO authorization for its bombing campaign.
Ban also questioned whether the use of force to deter Syria or other countries from deploying chemical arms in the future could cause more harm than good.
"I take note of the argument for action to prevent future uses of chemical weapons," he said. "At the same time, we must consider the impact of any punitive measure on efforts to prevent further bloodshed and facilitate a political resolution of the conflict."
"The turmoil in Syria and across the region serves nobody," he said. "I appeal for renewed efforts by regional and international actors to convene the Geneva conference as soon as possible."
'OUTRAGEOUS WAR CRIME'
The United States and Russia announced in May that they would organize an international peace conference on Syria to revive a stalled plan agreed in June 2012 in Geneva that called for a Syrian political transition and end to the violence. But neither the government nor rebels want to negotiate and plans for a new conference appear dead, diplomats say.
Ban said that if U.N. inspectors determine that chemical weapons were used in Syria, the Security Council, which has long been deadlocked on the civil war, should overcome its differences and take action.
"The Security Council has a duty to move beyond the current stalemate and show leadership," he said. "This is a larger issue than the conflict in Syria. This is about our collective responsibility to humankind."
Ban also reiterated that the use of chemical weapons of mass destruction is an international crime of the highest order.
"If confirmed, any use of chemical weapons by anyone under any circumstances will be a serious violation of international law and outrageous war crime," he said.
"Almost a century ago, following the horrors of the First World War, the international community acted to ban the use of these weapons of mass destruction," Ban said. "Our common humanity compels us to ensure that chemical weapons do not become a tool of war or terror in the 21st century."
"Any perpetrators must be brought to justice," he added. "There should be no impunity."
Ban said that samples and other evidence taken at the site of the attack in the suburbs of Damascus the United States says killed over 1,400 people, many of them children, would arrive at European laboratories on Wednesday. Ban told diplomats last week that analysis of those samples could take two weeks.
The United Nations has received at least 14 reports of possible chemical weapons use in Syria. After months of diplomatic wrangling, the U.N. experts, led by Swedish scientist Ake Sellstrom, arrived in Syria on Aug. 18 with a 14-day mandate to look for evidence.
The U.N. team was initially going to look into three incidents, but its priority became the Aug. 21 attack. The inspectors have also been looking into Syrian allegations that the rebels used chemical weapons three times last month against the Syrian army - allegations that Washington has dismissed.
Ban said Sellstrom's team would return to Syria to continue its investigation as soon as possible. The U.N. team will only determine whether chemical weapons were used, not who used them.