U.N. chief Ban Ki-moon warned Thursday of a "catastrophic civil war" in Syria after the massacre of more than 100 civilians in the town of Houla that sparked global horror and outrage.

"The massacres of the sort seen last weekend could plunge Syria into a catastrophic civil war, a civil war from which the country would never recover," he told an Istanbul forum of the U.N.-led Alliance of Civilizations initiative.

President Vladimir Putin warned Thursday that Russia's position on Syria would not shift under pressure despite the crisis likely topping the agenda during his upcoming visits to Berlin and Paris.

"Russia's position is well-known. It is balanced and consistent and completely logical," Interfax quoted Putin's spokesman Dmitry Peskov as saying.

Armed rebels have given the Damascus regime until noon (0900 GMT) Friday to stick to Kofi Annan's peace plan to end violence in Syria, warning they themselves will quit the truce unless the ultimatum is met.

"If the Syrian regime does not meet the deadline by Friday midday, the command of the Free Syrian Army announces that it will no longer be tied by any commitment to the Annan plan ... and our duty will be ... to defend civilians," a FSA statement said.

Senior U.S. senators John McCain and Joe Lieberman said Thursday it was time to arm Syria's opposition as they expressed disgust over a massacre last week blamed on government forces.

"It's time to act. It's time to give the Syrian opposition the weapons in order to defend themselves. It's not a fair fight," the Republican McCain told reporters in the Malaysian capital Kuala Lumpur.

Iran is using commercial aircraft to smuggle weapons and explosives to Syria and Hizbullah in Lebanon, the German broadcaster ZDF reported Wednesday.

Citing Western security sources, ZDF said that Iran Air and Yas Air have repeatedly used aircraft designated as passenger planes to transport weapons to the Assad regime and Hizbullah.

Prime Minister Najib Miqati on Thursday held three-hour talks in Istanbul with his Turkish counterpart Recep Tayyip Erdogan, urging Ankara to carry on with its “satisfactory and ongoing” efforts to secure the release of 11 Lebanese Shiite pilgrims abducted in Syria’s Aleppo on May 22.

“Mr. Erdogan stressed that Turkey will continue to exert intensified efforts to rescue the Lebanese citizens held in Syria,” Miqati’s press office said in a statement issued after the meeting.

Syria's Permanent Representative to the U.N. Bashar al-Jaafari reiterated that his country is confronting “terrorist groups” that are trained, protected and funded by some Arab and regional countries, including Lebanon.

“Some Arab and regional countries should stop providing these groups with funds, weapons, political support and media coverage,” Jaafari said on Wednesday in a press conference held after a U.N. Security Council session.

U.N.-Arab League Special Envoy for Syria Kofi Annan on Thursday held talks with President Michel Suleiman at the Baabda palace on his peace plan for strife-torn Syria and the issue of the 11 Lebanese pilgrims abducted on May 22 in the Syrian province of Aleppo.

During the talks with Suleiman, Annan “evaluated his plan for Syria and the stages it has gone through,” NBN television reported.

The Obama administration added new sanctions on a Syrian bank Wednesday as a top White House official said the U.S. wants to economically throttle the regime of Syrian President Bashar Assad and cut off salaries of pro-government thugs blamed for the grisly massacre in Houla.

The Treasury Department said the Syria International Islamic Bank has been acting as a front for other Syrian financial institutions seeking to circumvent sanctions. The new penalties will prohibit the SIIB from engaging in financial transactions in the U.S. and will freeze any assets under U.S. jurisdiction.

Western nations on Wednesday demanded increased pressure on Syria amid heightened outrage over two massacres in five days, but Russia maintained its opposition to sanctions.

The United States, France, Britain and Germany all came out of a U.N. Security Council meeting on the worsening crisis urging measures up to sanctions by the 15-nation body.