One of the key issues in the Syrian conflict over the past two years has been the supply of weaponry to both sides, which has kept the fighting going.
The government has been able to rely on a steady flow of arms from its foreign allies, while the rebels have received weapons and non-lethal aid in a more clandestine way. Here is a look at where the military support is coming from.
Who is arming the Syrian government?
Before the start of the uprising, the Syrian army had a range of heavy weapons, including tanks, armoured vehicles, artillery and rocket systems, and ballistic missiles.
The air force, meanwhile, had fighter jets and helicopter gunships at its disposal.
After two years of fighting, government forces are still better armed and organised than the rebels, but Western officials say their stockpiles of weapons and ammunition are being depleted and they have had to rely on foreign help.
Russia has continued to supply the Syrian military with weapons and equipment throughout the conflict. Moscow insists it is only fulfilling pre-existing contracts and that it is not violating any international sanctions.
Despite Western pressure, Moscow insisted earlier this year that it would be honouring its previously agreed contract with Damascus for supplying sophisticated S-300 surface-to-air missile defence systems. Although it is believed that the missiles are yet to be delivered to Syria.
Iran has stepped up its military support of Syrian government forces since the end of 2012, according to Western officials.
Tehran is believed to have become a key supplier of rockets, anti-tank missiles, rocket-propelled grenades and mortars.
However, Iranian officials deny breaking the UN sanctions imposed on its arms exports.
To evade the sanctions, Tehran has allegedly been transporting most of the weapons through Iraqi airspace on commercial planes and, more recently, overland through Iraq by lorry, something the Iraqi government denies.
Photographs and videos published online appear to provide evidence of recent Iranian arms shipments.
Syria's rebel groups are believed to have acquired their weapons and ammunition through a variety of means, including the black market, battlefield capture, improvised factories, and shipments paid for by individuals, groups and foreign governments.
Commanders have appealed for advanced weapons, but have reportedly received only limited shipments of small arms.
Representatives of the main rebel umbrella group, the Free Syrian Army (FSA), have said that the vast majority of its weaponry has been bought on the black market or seized from government facilities.
Rebel groups have captured a number of military bases since 2011, including at Atareb, Taftanaz, Jirah and Tiyas.
These have provided useful sources of ammunition and weapons, particularly anti-aircraft missile systems and armoured vehicles.
Until now, Qatar is widely believed to have been the main supplier of weapons to the rebels.
The Gulf emirate has denied providing any arms, although it has promised to support the opposition "with whatever it needs".
Most of the weapons are thought to have been given to hardline Islamist rebel groups, particularly those aligned with the Muslim Brotherhood, which has acted as an intermediary.
This has reportedly drawn criticism from Western officials who say many of the groups are extremist.
Qatar Emiri Air Force transporter planes flew to Turkey with supplies for the Syrian rebels as early as January 2012, according to the New York Times
By autumn 2012, Qatari aircraft were landing at Esenboga airport, near Ankara, every two days.
Qatari officials insisted they were carrying non-lethal aid.
Saudi Arabia is reported recently to have taken the lead in channelling financial and military support to the rebels.
Unlike Qatar, the Gulf kingdom is believed to be suspicious of the Islamist rebel groups, and has focused on supporting nationalist and secular factions of the FSA.
In late 2012, Riyadh is said to have financed the purchase of "thousands of rifles and hundreds of machine guns", rocket and grenade launchers and ammunition for the FSA from a Croatian-controlled stockpile of Yugoslav weapons.
These were reportedly flown - including by Royal Saudi Air Force C-130 transporters - to Jordan and Turkey and smuggled into Syria.
Saudi officials declined to comment.
The North African state has been a key source of weapons for the rebels.
The UN Security Council's Group of Experts, which monitors the arms embargo imposed on Libya during the 2011 uprising, said in April 2013 that there had been illicit transfers of "heavy and light weapons, including man-portable air defence systems, small arms and related ammunition and explosives and mines".
"The significant size of some shipments", it said, "and the logistics involved suggest that representatives of the Libyan local authorities might have at least been aware of the transfers, if not actually directly involved."
In May 2011, the European Union imposed an arms embargo on Syria.
As the uprising entered its third year, several member states - led by the UK and France - lobbied to be able to supply arms to "moderate" forces in the opposition.
Despite deep rifts, foreign ministers agreed to let the embargo lapse in May 2013.
Though EU member states do not appear to have already sent arms directly to the rebels, another European country has been linked to a secret, large-scale airlift.
In January 2013, a British blogger began to notice weapons made in the former Yugoslavia were appearing in videos and images posted online by rebels fighting in southern Syria.
The recoilless guns, assault rifles, grenade launchers and shoulder-fired rockets appeared to be from an undeclared surplus from the 1990s Balkan wars stockpiled by Croatia.
Western officials told the New York Times that the weaponry had been sold to Saudi Arabia, and that multiple planeloads had left Croatia since December 2012, bound for Turkey and Jordan.
They were reportedly then given to several Western-aligned FSA groups. Croatia's foreign ministry and arms-export agency have denied any such shipments occurred.
The US has repeatedly said it is reluctant to supply arms directly to rebel groups because it is concerned that weapons might end up in the possession of militant jihadist groups.
But on 14 June 2013 Washington said it would give the rebels "direct military aid" after concluding Syrian troops had used chemical weapons.
The CIA is reported to have played an important role behind the scenes since 2012, co-ordinating arms shipments to the rebels by US allies.
In June 2012, US officials said CIA officers were operating in Turkey, helping decide which groups would receive weapons.
The CIA is also reported to have been instrumental in setting up the alleged secret airlift of weapons from Croatia.
The Turkish government is a firm supporter of the rebels, but has not officially approved the sending of military aid.
However, reports suggest it has played a pivotal role in sharp acceleration of arms shipments to the rebels since late 2012.
The Turkish authorities had oversight over much of the airlift of weapons from Croatia, "down to affixing transponders to trucks ferrying the military goods through Turkey so it might monitor shipments as they move by land into Syria", according to the New York Times.
The Yugoslav-made weapons first seen in the hands of FSA units in southern Syria in early 2013 are believed to have been smuggled over the border with Jordan .
The Jordanian government has denied any role and said it was trying to prevent smuggling.
However, the New York Times found evidence to suggest Royal Jordanian Air Force transport planes and Jordanian commercial aircraft had been involved in the alleged airlift of arms from Croatia.
Syria's rebels, who are drawn mostly from the country's majority Sunni community, are said to have acquired weapons, ammunition and explosives from Sunni tribesmen and militants in neighbouring Iraq.
Arms are reportedly smuggled over the long, porous border and sold or given to the rebels. . Al-Qaeda in Iraq played an active role in founding the al-Nusra Front and provides it with money, expertise and fighters.
As with Iraq, Lebanon's Sunni community is reported to have helped supply Syrian rebel fighters with small arms purchased on the black market or shipped from other countries in the region, including Libya.
The Lebanese authorities have seized unmarked shipments of ammunition, including rocket-propelled grenades.
The Syrian town of Qusair, which was recaptured by government forces in June 2013, was a transit point for weapons smuggled from north-eastern Lebanon.