UN chief Ban Ki-moon has told world leaders at the G-20 summit that there is “no military solution” to Syria. Meanwhile, a U.S. ambassador to the UN accused Russia of holding the Security Council "hostage" over the crisis, according to Deutsche Welle.

UN Secretary-General Ban Ki-moon addressed world leaders on Thursday, Sept 5 during a closed-door dinner at the G-20 summit outside St. Petersburg saying the situation in Syria “has no military solution,” stressing the need for a political solution instead.

Ban urged the permanent members of the UN Security Council – China, France, Russia, the United Kingdom and the United States - that they have a “collective responsibility to mankind” to act. However, he reiterated that any decision “should be taken within the framework of the UN Charter, as a matter of principle.”

Ban earlier said that the use of force is only legal when it is in self-defense, or with Security Council authorization. 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.

"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."

With U.S. President Barack Obama hoping to use the two-day Group of 20 gathering in St. Petersburg to shape an international consensus behind his call for a military assault aimed at deterring Syrian leader Bashar al-Assad from deploying chemical weapons in the future, the U.S. Congress is divided over authorizing a strike and the global community seems equally uncertain about what action to take.

Italy's leader cautioned that a strike might lead to wider conflict. The head of the European Council ruled out a military solution. Germany is pushing to act through the International Criminal Court, the Wall Street Journal says.

From Rome, Pope Francis urged leaders of the G-20 nations, the world's largest economies, to avoid the "futile pursuit" of military action.

Italian Prime Minister Enrico Letta said some world leaders doubted Mr. Obama would be able to limit military engagement to his stated aim of carrying out punitive strikes against the Assad regime for what the U.S. says is its use of chemical weapons in a massacre outside Damascus on Aug 21.

"There are some who interpret this as the start of something of which we don't know the end," Letta said. "These are obviously two very different scenarios, and I'm sure that's part of what's blocking things and creating so much difficulty."

France is one of the few Western allies supporting a possible strike. President François Hollande hopes to get European leaders to coalesce around a summit statement Friday denouncing the Assad regime over chemical weapons.

"It's really important that the Europeans present at the G-20 are on the same page in condemning the use of chemical arms and condemning the regime that used them," Hollande said.

British Prime Minister David Cameron said Thursday that UK scientists have found evidence that poison gas was used in the alleged attacks.

In an interview with BBC television, Cameron said that the evidence "further shows the use of chemical weapons in that Damascus suburb."

Despite the new evidence, Britain will not join a U.S.-led military strike as the UK parliament voted down a bid by Cameron for military intervention. However, Washington has found a firm partner in France. China has already expressed its "grave concerns" over unilateral military strikes.

"War cannot solve the problem in Syria," Chinese delegation spokesman Qin Gang told reporters at the G20. German Chancellor Angela Merkel has repeatedly ruled out her country's participation in any U.S.-led military strike against Assad's regime.

Obama's toughest obstacle is Russian President Vladimir Putin. Frustrated by Russia's lack of cooperation on Syria, as well as Moscow's decision to grant former NSA contractor Edward Snowden temporary asylum, Obama canceled a meeting with Putin that had been scheduled in advance of the G-20.

Putin has been among the loudest critics on the international stage of Obama's push for a military strike in Syria. He blasted the push as an "act of aggression." He has said in recent interviews that a strike would be illegal if the United Nations does not support it.

The Russian President called U.S. Secretary of State John Kerry a liar, claiming he had denied that al-Qaeda was fighting with the Syrian opposition in that country's civil war.

Speaking to his human rights council, Putin recalled watching a congressional debate where Kerry was asked about al-Qaeda. Putin said he had denied that it was operating in Syria, even though he was aware of the al-Qaeda-linked Jabhat al-Nusra group.

Putin said: "This was very unpleasant and surprising for me. We talk to them (the Americans) and we assume they are decent people, but he is lying and he knows that he is lying. This is sad."