Russia has nothing to add to the political debate on Syria in the United States, a White House official said on Friday, also signaling that Washington is ready to take punitive action against the Syrian regime without the U.N. Security Council's backing.

"I don't know that the Russians have anything to add to the debate in the United States, given that we know where Russia stands on this issue," said Ben Rhodes, President Barack Obama's deputy national security adviser for strategic communications, after senior Russian lawmakers expressed a desire to visit Washington and persuade Congress not to approve military action against Syria.

Russia on Friday warned the United States against targeting Syria's chemical arsenal as Washington considered the use of force against President Bashar Assad's regime.

"With particular concern we perceive the fact that military infrastructure facilities securing the integrity and safety of Syria's chemical arsenal are among the possible targets for military strikes," the Russian Foreign Ministry said in a statement.

U.N. chief Ban Ki-moon made an impassioned plea Friday against military action in Syria, warning that it could spark further sectarian violence in a country already suffering from a humanitarian crisis "unprecedented" in recent history.

Speaking at a humanitarian meeting hosted by Britain on the sidelines of the G20 summit, Ban called on world powers to put aside their differences over the Syrian conflict, and to take concerted action to get desperately needed aid to the population.

Syrian rebels who seized an entry to a Christian town north of Damascus this week have now withdrawn to protect religious and archaeological sites there, an opposition statement said.

"Free Syrian Army (FSA) units on Wednesday destroyed posts at Maalula and Jabadine held by the army on the Damascus-Homs road after fierce clashes with President Bashar Assad's forces and auxiliaries," the Syrian National Coalition said overnight.

Russian President Vladimir Putin discussed the situation in Syria with British Prime Minister David Cameron early on Friday, Putin's spokesman said.

Cameron and Putin met on the sidelines of the G20 summit in Saint Petersburg after discussing the Syrian crisis over dinner with other world leaders and watching a cultural show that ended around 2:00 am (2200 GMT).

Iran is carefully watching the divisive U.S. debate on whether to launch military strikes against its chief ally, Syria, but the Obama administration may be at risk of sending Tehran the wrong message, analysts warn.

In days of passionate testimony, U.S. Secretary of State John Kerry said America and the world must send a warning to Iran and others that they will not turn a blind eye to the use of non-conventional arms amid accusations the regime of Syrian President Bashar Assad unleashed chemical weapons on his people.

World leaders at the G20 summit on Friday failed to bridge their bitter divisions over U.S. plans for military action against the Syrian regime, with Washington signaling that it has given up on securing Russia's support at the UN on the crisis.

A dinner hosted by Russian President Vladimir Putin that ran on into the early hours of the morning failed to win a breakthrough on how to halt a conflict in Syria that has claimed more than 100,000 lives and which is now in its third year.

Algerian soldiers killed two armed Islamists on Thursday in a military operation east of the capital, the defense ministry said.

The incident occurred in Ain El Hamra, in the Boumerdes region, with a weapons cache recovered in the operation, the ministry added, without elaborating.

German Foreign Minister Guido Westerwelle on Thursday said that the International Criminal Court should examine the use of chemical weapons in Syria.

"I have again pressed for the U.N. Security Council to give a mandate to the ICC to examine the chemical attack in Syria," he said on the sidelines of the G20 summit in Russia's Saint Petersburg.

NATO chief Anders Fogh Rasmussen sharply regretted international divisions over Syria on Thursday, warning that such splits risked sending a message of encouragement to dictators across the globe to use prohibited deadly weapons.

"I strongly regret that division within the international community," Rasmussen told Agence France Presse on the sidelines of an EU defense ministers' meeting taking place in Lithuania as the United States and Russia sought to overcome divisions over a U.S.-led push for military action against the Damascus regime.