U.N. humanitarian chief Valerie Amos arrived in the Syrian capital Damascus on Thursday for talks with government officials, a U.N. source told Agence France Presse.

Amos' visit comes shortly after the United Nations announced that the number of Syrian refugees had passed the two million mark since the country's conflict began in March 2011.

John Boehner, the top Republican in the U.S. House of Representatives, has rejected a request to meet a Russian delegation to discuss Syria, his spokesman said Wednesday.

Earlier this week, Russian President Vladimir Putin publicly endorsed plans to send a parliamentary delegation to the United States for talks with Congress.

Leaders of the Roman Catholic Church in the United States opposed military action against Syria on Wednesday, saying it would led to "unintended negative consequences."

In a letter to President Barack Obama, the U.S. Conference of Catholic Bishops said Washington should instead "work urgently and tirelessly" with others to end Syria's drawn-out civil war.

Washington is weighing expanding support for Syrian rebels by having the Pentagon take charge of arming the opposition instead of a clandestine effort by the CIA, officials said Wednesday.

"It's under consideration," said a U.S. official, who spoke on condition of anonymity.

U.S. President Barack Obama's plan to conduct punishing military strikes on Syria passed its first congressional hurdle Wednesday, paving the way for full Senate debate on the use of force.

The Senate Foreign Relations Committee passed an amended resolution 10-7, with one senator voting present, that authorizes U.S. military intervention with a 90-day deadline and bars U.S. boots on the ground for combat purposes.

An international coalition for action against Syria is growing, Secretary of State John Kerry told U.S. lawmakers Wednesday, adding that Arab nations had even offered to pay for any strikes.

"We are building support with ... other countries, among them the Arab League," Kerry said, as the U.S. administration tried to sell its plan for military action to punish Syria for its use of chemical weapons.

An international political grouping headed by former U.N. secretary general Kofi Annan came out against military action in Syria Wednesday, as the U.S. sought allies for strikes following alleged chemical attacks.

"There is no military solution to this conflict," said The Elders, a group founded by former South African president Nelson Mandela.

Limited military strikes against Syria favored by President Barack Obama may have a psychological effect but little practical impact on the Damascus regime's firepower, experts said Wednesday.

Vowing to avoid large-scale, open-ended action, Obama has proposed a "shot across the bow" in a bid to punish President Bashar Assad for allegedly resorting to chemical weapons.

Russian naval vessels in the Mediterranean are capable of reacting to an escalation in the Syria conflict, a military source said Wednesday, as Moscow fine tunes its maritime presence ahead of possible U.S. military action.

"Today we consider our presence in the eastern Mediterranean to be sufficient to solve the tasks. If necessary, together with submarine forces, they (the ships) are capable even today of influencing a military situation," a general staff source told the Interfax news agency.

French Prime Minister Jean-Marc Ayrault urged lawmakers Wednesday to back military action against the Syrian regime in response to "the most massive and terrifying use" of chemical weapons this century.

The debate came as Syria's deputy foreign minister Faisal Muqdad ridiculed France as an American stooge.