The United States Wednesday denied Russia's claim that it is arming Syria's opposition and expressed new concern over what it says is Moscow's supply of attack helicopters to Damascus.

"We do not and have not supplied weapons to the Syrian opposition. You know our position on that and we have made it very clear," White House spokesman Jay Carney told reporters.

Russia's arms export agency said Wednesday its deliveries to Syria were in line with U.N. regulations but declined to comment specifically on U.S. claims that it was sending attack helicopters to the regime.

Rosoboronexport "does not supply weapons and military technology in contradiction with U.N. Security Council security requirements and other international agreements," an agency spokesman told the state-run RIA Novosti news agency.

Secretary of State Hillary Clinton warned that Washington would find it hard to support the extension of the observer mission beyond next month if the government of President Bahar al-Assad's regime continued to show "contempt" for a U.N.-backed ceasefire that was supposed to go into effect from April 12.

Asked whether he believed Syria is in a civil war, Ladsous told reporters: "Yes I think we can say that. Clearly what is happening is that the government of Syria lost some large chunks of territory, several cities to the opposition, and wants to retake control.

World powers will outline to Iran a "very clear path" to resolve the impasse over its suspect nuclear program at talks in Moscow next week, U.S. Secretary of State Hillary Clinton said Tuesday.

"There is a unified position being presented by the P5+1 that gives Iran, if it is interested in taking a diplomatic way out, a very clear path that would be verifiable and would be linked to action for action," Clinton told a U.S. think-tank.

Afghan President Hamid Karzai on Tuesday said NATO had agreed not to carry out air strikes on residential areas even in self-defense, apparently contradicting comments made by senior coalition commanders.

NATO's International Security Assistance Force (ISAF) ordered an end to air strikes on homes except as a last resort to ensure the defense of troops, Lieutenant General Curtis Scaparrotti, deputy commander of U.S. forces, said on Monday.

Syria on Tuesday accused Washington of encouraging more massacres in the strife-torn country and of meddling in its internal affairs.

"The U.S. administration is pushing forth with its flagrant interference in Syria's internal affairs and its backing of armed terrorist groups," the foreign ministry said in a statement.

Beijing said Tuesday China's purchases of Iranian oil were legal and transparent, after the United States refused to exempt it from tough new sanctions.

Washington announced Monday that tough new sanctions that penalize foreign financial institutions would not be imposed on seven emerging economies, including India, after those countries cut back on the amount of oil they buy from Iran.

An alleged Libyan jihadist group has claimed responsibility for last week's bombing of the U.S. consulate in Libya's main eastern city of Benghazi, the SITE monitoring service said Monday.

The "Brigades of the Imprisoned Sheikh Omar Abdul Rahman" said the attack was to avenge the death of Al-Qaeda number two Abu Yahya al-Libi in a drone strike in Pakistan, said the US-based SITE, which monitors jihadi websites.

The United States said Monday it would exempt seven countries including India from tough new sanctions on their banking sector after they cut oil purchases from Iran.

Secretary of State Hillary Clinton added India, Malaysia, South Africa, South Korea, Sri Lanka, Turkey and Taiwan to countries exempt from the new sanctions. In March, she made exemptions for European Union nations and Japan.

The United States on Monday voiced concern after Russian police raided the homes of top protest leaders ahead of a planned mass rally in Moscow against President Vladimir Putin.

"The United States is deeply concerned by the apparent harassment of Russian political opposition figures on the eve of the planned demonstrations on June 12," State Department spokeswoman Victoria Nuland told reporters.