U.S. President Barack Obama will again make the case for military action in Syria when he speaks with key lawmakers.
Mr. Obama is scheduled to meet with prominent Senator John McCain at the White House Monday. McCain has long urged the president to take forceful action against Syria, possibly even removing Syrian President Bashar al-Assad from power.
But many U.S. lawmakers remain skeptical, questioning whether the military should be involved at all.
U.S. officials Sunday briefed some lawmakers on intelligence showing the Syrian military dropped poison gas on civilians outside Damascus last month, killing more than 1,000 people.
International support for military intervention in Syria, despite evidence of a possible chemical weapons attack, has been waning. But NATO Secretary General Anders Fogh Rasmussen said Monday something must be done.
"It is my firm position that the international community should react in such a situation, otherwise we would send the very dangerous signal to dictators all over the world that they can use a chemical weapons without any reaction from the international community.''
So far, only France and Turkey have come out strongly in favor of a military response.
Russia, a long-time Syrian ally, questioned Monday the credibility of U.S. evidence the Assad government used chemical weapons.
Russian Foreign Minister Sergei Lavrov also cautioned that any military strike will only make it more difficult to find a political solution.
"If the action announced by the U.S. President, to our great disappointment, does take place, then I think, regardless of all words about the Geneva-2, it will delay any prospects for such forum for a long time, if not forever.''
Arab League Secretary General Nabil Elaraby also warned Monday a military option in Syria is "out of the question" though he reiterated those behind the alleged chemical weapons attack must answer for their actions.
"I cannot say that there are two viewpoints (within Arab states), however I can say that there is a general opinion represented by 18 countries, in which what is needed is to take deterring measures against those who committed this crime (using chemical weapons).''
The Syrian government denies using chemical weapons and says the rebels have used poison gas against Syrian troops.
Its state-run SANA news agency said Monday that Syrian ambassador to the U.N. Bashar Ja'afari has sent a letter urging Mr. Ban to "prevent any aggression" against Syria.
Meanwhile, the United Nations refugee agency says much more money is needed to help the ever-growing number of Syrians forced to flee the fighting.
Tarik Kurdi told the Associated Press nearly one-third of Syria's pre-war population has now been forced to flee.
"According to the estimations of the United Nations there are about 7 million people affected by the crisis, of those about five million have been displaced from their cities and villages to somewhere else inside Syria. The majority of them are women and children."
The UNHCR says it has only received about $70 million of the $250 million it needs to help the refugees.