By Jonathan Saul and Parisa Hafezi

LONDON/ANKARA (Reuters) - As Syria's war nears the start of its fourth year, Iran has stepped up support on the ground for President Bashar al-Assad, providing elite teams to gather intelligence and train troops, sources with knowledge of military movements say.

This further backing from Tehran, along with deliveries of munitions and equipment from Moscow, is helping to keep Assad in power at a time when neither his own forces nor opposition fighters have a decisive edge on the battlefield.

Assad's forces have failed to capitalize fully on advances they made last summer with the help of Iran, his major backer in the region, and the Hezbollah fighters that Tehran backs and which have provided important battlefield support for Assad.

But the Syrian leader has drawn comfort from the withdrawal of the threat of U.S. bombing raids following a deal under which he has agreed to give up his chemical weapons.

Shi'ite Iran has already spent billions of dollars propping up Assad in what has turned into a sectarian proxy war with Sunni Arab states. And while the presence of Iranian military personnel in Syria is not new, military experts believe Tehran has in recent months sent in more specialists to enable Assad to outlast his enemies at home and abroad.

Analysts believe this renewed support means Assad felt no need to make concessions at currently deadlocked peace talks in Geneva.

MILITARY SPECIALISTS

Assad is now benefiting from the deployment by Tehran of hundreds more military specialists to Syria, according to Iranian sources familiar with deployments of military personnel, Syrian opposition sources, and security experts.

These include senior commanders from the elite Quds Force, the external and secretive arm of the powerful Islamic Revolutionary Guard Corps, as well as IRGC members.

Their function is not to fight, but to direct and train Syrian forces and to assist in the gathering of intelligence, according to sources in Iran and outside.

Story continues

An Iranian foreign ministry official said: "We always have said that we support our Syrian brothers and respect their will ... Iran has never got involved in Syria by providing arms or financially or by sending troops."

But a former senior Iranian official with close IRGC links said Iranian forces were active in Syria.

He said the Quds force was gathering intelligence in Syria, which Iran regarded as a top priority. He said a few hundred commanders from the Quds Force and the IRGC were in Syria, but they did not get involved directly in the fighting.

A recently retired senior IRGC commander said Iranian forces on the ground included some Arabic speakers. He said top Quds force commanders numbered 60 to 70 at any given time.

These men were tasked with advising and training Assad's military and his commanders, he said. Revolutionary Guards directed the fighting on the instructions of the Quds Force commanders, he added.

The former IRGC commander said these personnel were also backed up by thousands of Iranian paramilitary Basij volunteer fighters as well as Arabic speakers including Shi'ites from Iraq. The former Iranian official and a Syrian opposition source also put auxiliary forces in the thousands.

The figures could not be independently verified from Syria, but the deaths of at least two IRGC commanders in Syria have been publicly reported.

European and U.S. security officials said hundreds of Iranians were active in Syria, advising, training and in some cases commanding Syrian government forces.

"Iran's presence in Syria has been and remains a concern, given the resources Tehran has at its disposal and its unwavering support for the Assad regime," a U.S. official said.

A report by Western intelligence agency obtained by Reuters said that while Assad forces had an advantage in the balance of fighting on the ground, at present they were "unable to translate this advantage into a decisive victory". The report said Assad's dependence on Hezbollah and Iran was increasing.

"Behind the regime's improved combat capabilities stands significant support from Hezbollah, Iran and Russia, while at the same time the rebels' weaknesses are becoming more pronounced," the report said.

"Syria is especially important to Iran and Hezbollah in geo-strategic terms - and its ability to act as a conduit to help strengthen the radical (i.e. pro-Iran and pro-Hezbollah) bloc."

Scott Lucas, of EA WorldView, a specialist website on Iran and Syria, said the evidence indicated hundreds of Iranian advisers and trainers were inside Syria at any one time.

"They are trying to work with the Syrians in ramping up the number of (Syrian) troops they can put in the field and making sure those guys can hold the line as well as carry out certain offensive operations," he said.

ACROSS BORDERS

Iranian and Syrian opposition sources said personnel could enter Syria through the border with Turkey since Iranians did not need visas to enter Turkey. Others come in across the Iraqi border, and more senior commanders fly in to Damascus.

A Turkish official said the number of Iranians crossing into Syria had increased in the last few months. Most had non-Iranian passports.

A Syrian opposition source said in recent months that Iranian- led forces had begun operating in coastal areas including Tartous and Latakia. They have local ID cards, wear Syrian military fatigues and work with the elite Syrian air force intelligence unit.

The presence of units in coastal areas could not be independently corroborated. The Iranian sources declined to give details of where the forces were located.

Reuel Marc Gerecht, a former Middle East case officer with the CIA, said Iran sought to avoid getting embroiled in direct fighting.

"It would be difficult to integrate Iranians into Arab combat operations, and they would essentially have to run their own combat operations, since they would be loath to put themselves under the Alawite control," said Gerecht, who is now with U.S. think-tank the Foundation for Defense of Democracies.

Assad is a member of the Alawite sect, an offshoot of Shi'ite Islam.

Torbjorn Soltvedt, of risk consultancy Maplecroft, said Iran's role in training and coordinating "constitutes a lifeline for the regime".

"The involvement of Iranian Revolutionary Guard personnel and Shiite militias such as Hezbollah remains crucial to the Syrian regime's war effort," Soltvedt said.

WEAPONS LIFELINE

In recent weeks Syria has continued to receive arms and military equipment from Russia and via proxies, according to several sources. Those supplies included unmanned spy drones, guided bombs and spare parts for combat craft.

Moscow says it violates no international laws with its military supplies to Syria, which do not include offensive weapons.

Nic Jenzen-Jones, a military arms specialist and director of Armament Research Services, said Iranian-made Falaq-1 and Falaq-2 rocket launchers had been sent from Iran to Syria.

"While they have been around for a while, we have seen an increase in use of late," he said

Jenzen-Jones added that relatively new Iranian small arms ammunition - produced in the last three to four years - had reached Syria recently.

A rebel fighter operating in Homs province with Islamist group Liwa al-Haq said opposition forces knew of Iranian planes flying into Hama airport in central Syria to deliver weapons.

A source in the international arms industry with knowledge of Middle Eastern weapons movements said Syria had received millions of rounds of ammunition for light weapons of late, much of it former eastern bloc material coming in by sea and air from the Black Sea area.

The Syrian opposition source said Latakia airport and port as well as the port in Tartous were used to bring in equipment.

Other supplies included machine guns and ammunition for artillery and tanks, the arms industry source said.

(Additional reporting by Erika Solomon in Beirut and Louis Charbonneau in Vienna, Editing by Giles Elgood and Will Waterman)