US-led coalition forces have parachuted ammunition to Syrian rebels fighting the Islamic State of Iraq and the Levant (ISIL) group, according to a US official.
Elissa Smith, a spokesperson for the US defence secretary, said on Monday that the airdrop took place in northern Syria on Sunday.
"This successful airdrop provided ammunition to Syrian groups whose leaders were appropriately vetted by the United States and have been fighting to remove ISIL from northern Syria," she said.
"The airdrop includes small arms ammunition. Due to operational security we will not have any further details about the groups that received these supplies, their location, or the type of equipment in the airdrop."
Meanwhile, Russia - which believes it is wrong of US to arm the groups that Washington calls "moderate" - intensified its air strikes on Monday in central Syria.
The Russian defence ministry said it had struck 53 alleged ISIL targets in the past 24 hours, destroying command centres, ammunition and fuel depots, as well as training camps allegedly used by foreign fighters.
The ministry said the ISIL positions were in the central provinces of Homs and Hama, as well as in Latakia and Idlib.
Russia insists it is mainly targeting the ISIL group and other "terrorists," but the multi-pronged ground-and-air offensive is being waged in areas controlled by mainstream rebels as well as al-Nusra Front.
Human Rights Watch has echoed accusations by Syrian activists that Russia was behind the use of new advanced cluster munitions in Syria by dropping them from warplanes or supplying them to the Assad government.
The Pentagon says the vast majority of Russian strikes have been against opponents of President Bashar al-Assad, whose forces have made gains since Russian military intervention on September 30.
Government forces have pushed to regain the Sahl al-Ghab plain, which is adjacent to Latakia province, the heartland of Assad's regime. Now the Syrian army is focusing its fight on the village of Kafr Nabudeh.
Capturing Kafr Nabudeh would cut off a major highway, giving the pro-government forces access to the northwestern province of Idlib.
Against this backdrop, Syrian Kurdish forces fighting ISIL have said they are officially uniting with a mix of rebel groups.
The leaders of the new military alliance - the Democratic Forces of Syria - said they have the backing of both US and Russia.
Amid continued US-Russian rivalry, the UN envoy, Staffan de Mistura, on Monday called on Moscow and Washington for urgent agreement on Syria.
He said he would travel on to Washington "immediately after" his Moscow visit, as he struggles to set up so-called contact groups of countries with interests and influence in the Syria conflict that could help it move towards a political solution.
About 250,000 lives have been lost since the uprising began in 2011 and nearly half of the population is displaced, with millions of them fleeing out of the country for safety.
For more than a year, the US is leading a coalition of some 60 nations that has carried out more than 7,000 drone and air strikes against ISIL, which controls large parts of Syria and neighbouring Iraq.
The Pentagon was forced to scrap plans to train thousands of Syrian rebels in Turkey and Jordan after many failed to pass the screening process and one group of graduates gave some of their ammunition and other gear to an al-Qaeda affiliate.
Source: Al Jazeera and agencies