U.S. President Barack Obama arrived on Wednesday in Saint Petersburg for the G20 summit in Russia, where officials said he would argue his case for military action against the Syrian regime over an alleged chemical weapons attack.

With a serious expression, Obama jogged down the steps off his Air Force One plane which touched down at Saint Petersburg's Pulkovo airport ahead of the opening by Russian President Vladimir Putin of the summit later in the afternoon.

Russia said Thursday it would warn a meeting of the U.N.'s atomic watchdog next week that any U.S. military strikes in Syria could hit a nuclear research reactor there.

"Russia will for sure raise this topic at the autumn session of the board of governors of International Atomic Energy Agency (IAEA) opening on September 9 in Vienna," a foreign ministry spokesman told Agence France Presse.

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.

Mexican President Enrique Pena Nieto said Wednesday he would voice concerns about alleged U.S. spying on his emails to U.S. counterpart Barack Obama, warning it would be illegal if proven true.

The two leader will come face to face at the Group of 20 meeting in Russia on Thursday, four days after a U.S. journalist reported that the National Security Agency snooped on the online activities of the Brazilian and Mexican leaders.

U.S. President Barack Obama Wednesday urged world support for punitive strikes against Syria for its alleged use of chemical weapons, while Damascus vowed retaliation and resistance even if a third world war erupts.

Obama, fresh from efforts in Washington to secure bipartisan support for military intervention, said in Stockholm that the world had set "a red line" for Syria and it could not now remain silent in the face of the regime's alleged use of chemical weapons.

The Council of Maronite Bishops condemned on Wednesday the alleged use of chemical weapons in Syria but rejected the use of force, saying the war should be resolved peacefully.

The war in Syria is “taking dangerous turns” since the alleged use of chemical weapons and threats to strike the country, said the bishops following their monthly meeting under Patriarch Beshara al-Rahi at his summer residence in Diman.

President Vladimir Putin warned the West against taking one-sided action in Syria but also said Russia "doesn't exclude" supporting a U.N. resolution on punitive military strikes if it is proved that Damascus used poison gas on its own people.

In a wide-ranging interview with The Associated Press and Russia's state Channel 1 television, Putin said Moscow has provided some components of the S-300 air defense missile system to Syria but has frozen further shipments. He suggested that Russia may sell the potent missile systems elsewhere if Western nations attack Syria without U.N. Security Council backing.

America's biggest pro-Israel groups are throwing their weight behind President Barack Obama's plan for U.S. military intervention in Syria.

The American Israel Public Affairs Committee, known as AIPAC, said Tuesday that congressional authorization for strikes against Syrian President Bashar Assad's regime would ensure that what it called "barbarism on a mass scale" does not go unanswered.

The World Bank is helping Lebanon prepare the ground to request an influx of international aid to offset the high costs of the spillover from the Syrian civil war, President Jim Yong Kim told The Associated Press on Tuesday.

Kim said in an interview that the Lebanese government asked the World Bank to take the lead in preparing a quick assessment of the social and economic impacts of the war in neighboring Syria. This analysis will be presented during a Sept. 25 meeting of an international support group for Lebanon at the United Nations General Assembly.

The Senate resolution authorizing U.S. President Barack Obama to use military force against Syria would bar American ground troops for combat operations and set a deadline for any action.

The Associated Press obtained a copy of the draft resolution that the Foreign Relations Committee will vote on Wednesday.