BEIRUT - Syrian rebels were preparing for an imminent ground assault by the army on the last enclave they hold in Damascus after air strikes and shelling increased on Tuesday, an insurgent spokesman said.Jobar district in northeast Damascus and the adjoining Eastern Ghouta region of towns and farms, east of the capital - designated as a de-escalation zone two weeks ago - has been under opposition control for much of the six-year conflict.The Failaq al-Rahman rebel group expected the army to attack the enclave at two positions it holds: Tayba, northeast of Jobar, and Ain Terma, southeast of it."We expect there will be attempts to assault in a few hours because an assault usually starts with a sweeping bombardment," a spokesman for the rebel group said.Rebels and other witnesses said shelling and air strikes increased on Monday and the bombardment was at its heaviest in a two-month Syrian army campaign.People had retreated to shelters, one resident said, noting that at least five buildings had collapsed under the shelling in two days.Many locals have left the area over recent weeks because of the bombardment and, even if the army assaults, they will still be able to do so, the resident said."This isn't Aleppo, where people were surrounded. Ghouta is a bigger space and the towns are more open to each other. No town is surrounded."However, there was little movement between Eastern Ghouta towns because of fighting over territory between rival rebel groups who control that area.The spokesman for Failaq al-Rahman, one of the largest rebel groups in that part of Syria, said 400 fighters had defected to its ranks in the past week from Ahrar al-Sham, one of the other main insurgent groups.