DAMASCUS: Syria is calling up former soldiers from the reserves to active army service in growing numbers, a sign of the strain of efforts to crush the 17-month-old revolt against President Bashar Assad.

Several fleeing reservists and a serving army officer told Reuters that thousands of men had been called up in the past two months to bolster the 300,000 strong army, and many of them are failing to report for duty.

"We have two choices: Stay and kill fellow Syrians, or desert, and be on the run from military courts," said a legal assistant summoned for duty in Damascus. Like others interviewed for this article, he asked not to be identified for security reasons.

One army officer contacted in Homs said he believed that only half of those called up in recent months had reported for duty, although it was not possible to verify that figure or ascertain whether other units had experienced similar levels of reservists failing to report.

The officer said many units had suffered heavy losses battling rebels.

"There is a shortage of men. A lot of fighters have been killed, and we have desertions," he said by telephone, sighing.

Most Syrian men are required to serve in the army for two years when they turn 18 or after finishing university. After a man has served, he remains in the reserves and can be called up for active duty.

Syria's conflict has killed more than 20,000 people. Fleeing reservists said that whatever their political stance, they did not want to be part of the country's civil war.

The fighting has intensified in the past two months, with rebels, often led by army defectors, launching advances in the capital Damascus and commercial hub Aleppo despite being massively outgunned by one of the region's best-equipped armies.

Syrian authorities, who say they are fighting foreign-backed terrorists, have not given full details of military casualties. One anti-Assad monitoring group, the Syrian Observatory for Human Rights, says nearly 6,000 soldiers and members of the security forces have been killed.

The Homs officer said reservists had been called up for several months but demand had risen in the past two months, especially since the surge in fighting in Damascus and Aleppo.

"We have yet to need full army mobilization. But if the situation deteriorates in the coming months, we may need it. The country is in a state of war and we need everyone's help."

Residents in Damascus say checkpoints across the city now inspect young men's IDs to check they are not fleeing army service or have not been called up from the reserves. Some deserters dare not leave their homes, fearing neighbors who might report them.

The legal assistant, who became a reservist after finishing his required military service in Syria's special forces two years ago, said he was stopped at a checkpoint in the capital and taken to an army reserve centre outside Damascus for a two-week training session.

He said he ran away from his training camp one night, and is now in hiding.

Syrian law requires men who served in the military to get army approval for passports, state jobs and even marriage licenses, which makes it more difficult for reservists to avoid a call-up.

Fadi, a former artillery specialist, said he was called by the army for active duty and given 48 hours to prepare to leave his coastal city of Tartous.

"I was terrified. I don't want my baby daughter to grow up fatherless. My wife is crying non-stop. If I have to be on the run for the rest of my life, I won't report for duty," he said.

A member of Assad's minority Alawite sect, Fadi, 30, is trying to find a way to bribe a security officer to let him flee the country.

Many Alawites like Fadi have stood by Assad, fearing sectarian retribution from the Sunni Muslim majority leading the revolt. "If my community found out what I was trying to do, they would call me a traitor," he said. "No one would help me hide."

The army officer in Homs said men under 30 or men who had recently completed their army service were being called up by military headquarters first, as well as men who had specialized in artillery or armored vehicles units.

Even opponents of Assad have been called up. Tamouz, a 28-year-old playwright who was arrested earlier this year for opposition activism, said he was called up for military service last week and fled the next day.

"I did my service in the infantry," he said. "Nowadays, that basically means: 'Go, die.'"

Syrian state television shows video loops of young soldiers shooting their weapons and marching in training drills to the sound of the national anthem, "Protectors of the Home."

The legal assistant said that before he escaped he had trained with 200 conscripts from all around the country.

"The officer training us tried to raise our spirits, he would smile and play patriotic songs. Some people seemed excited, but most of us were scared and felt deflated."

Once neutral, he said he was forced to pick sides in the conflict after his callup, and is now working with the opposition. "Why should we spend our whole lives serving the Assad family?"