By Tom Perry and Suleiman Al-Khalidi
BEIRUT (Reuters) - Momentum that has swept Syrian rebels to a series of swift victories over President Bashar al-Assad appears to have slowed in two major attacks aimed at capturing the cities of Aleppo in the north and Deraa in the south.
The reasons include a determined military effort by the government and its foreign allies to stem the tide of setbacks, and shortcomings in rebel strategy and weaponry that have made it harder for them to capture two cities of strategic value.
This does not necessarily mean Assad, his sway mostly confined to western Syria, is on the cusp of a major fightback. The army and allied militia remain overstretched in a multi-sided war pitting them against Islamic State, al Qaeda, and more moderate rebels.
But rebels say the going is proving harder than expected in cities where government forces are well entrenched and drawing on air power which the insurgents have lacked the means to confront throughout the four-year-long war.
"The battle is difficult and the regime is well fortified," said Bashar al-Zoubi, head of one of the rebel groups battling for Deraa. "It will take a very long time because the regime is fortified among civilians," he told Reuters.
Since late March, Assad has lost nearly all the northwestern province of Idlib to an Islamist alliance including al Qaeda's Nusra Front and important areas of the south including a border crossing and a garrison town to more moderate rebels.
The ultra-hardline Islamic State has also advanced into government-held areas, seizing Palmyra in central Syria - the first time the group has taken a city from Assad.
Seeking to build on their gains, the southern rebels launched the attack to drive remaining government forces from Deraa city in late June. Like Aleppo, Deraa city is already partially under rebel control and largely surrounded by insurgent-held countryside.
"THE OPERATION CONTINUES"
But unlike other recent battles where rebel advances have forced government retreats, the army and militia fighting alongside it have stayed put in Deraa, where tens of thousands of people still live.
The army had deployed heavy air strikes, including the use of barrel bombs, said Zoubi, whose Yarmouk Army is part of the Western-backed Southern Front alliance. "The civilians are the ones suffering."
He said the rebels had recently resubmitted a longstanding request for anti-aircraft missiles, but foreign states that back them had given no answer. "The operation continues," he said.
The Southern Front groups, widely assessed as the main rebel force in the south, have sought to exclude the al Qaeda-linked Nusra Front from the operation. Nusra had responded by trying to obstruct the assault, Southern Front spokesman Isaam al-Rayyes said. "This had an impact."
For Assad, Deraa is more strategically important than other areas lost recently. Were the well-organized Southern Front groups to take the city, it would hasten their advance towards Damascus, just 100 km (60 miles) to the north.
A senior Middle Eastern official close to Damascus said increased Russian and Iranian military and financial support had helped Assad, his government drained by the conflict. "Iranian and Russian support is increasing to the regime," he said.
Deraa, heavily garrisoned because of its proximity to Israel, is defended by elite army units, the source said.
"We must also say the militants did not unite in Deraa as they did in other places such as Idlib, which weakened them. The same can be said for Aleppo."
The rebels' difficulties in Deraa have illustrated the limitations to the support offered by foreign states that have supplied them with military aid via Jordan. The rebels say most of their weapons are seized from the Syrian army.
But foreign support has helped them maintain the upper hand over jihadists, preventing al Qaeda or Islamic State from taking over areas at the border with Jordan and Israel.
IRANIAN SUPPORT FOR ASSAD
Western officials hope the increased military pressure on Assad could help bring him to negotiations to end the conflict that has killed close to a quarter of a million people and triggered the worst refugee crisis since World War Two.
But sources close to Assad forecast a protracted conflict. Lebanon's Hezbollah, an Iranian-backed group fighting alongside Assad, is battling insurgents in areas near Damascus.
And there have been renewed pledges of support from Iran, which has helped to mobilize foreign Shi'ite militias to fight in Syria. Senior Iranian official Ali Akbar Velayati said in a recent newspaper interview that Muslims from countries such as Afghanistan would help "prevent Syria from falling".
Aleppo, 400 km (250 miles) from Damascus, may be a more remote concern than Deraa for Assad, but the rebels there report similar difficulties in advancing into heavily defended areas since they launched their biggest assault there in three years.
"The regime has put all its strength in Aleppo. It's a major center - four or five times the size of Idlib," said Abu Asad Dabeq, military commander of al-Failaq al-Awal, a rebel group fighting under the banner of "The Free Syrian Army".
The insurgent attack on Aleppo has also been hampered by rivalry between jihadist groups and more moderate forces.
Two separate command centers were announced at the start of the attack. One of them included the Nusra Front and said its stated goal was to implement Islamic sharia law in Aleppo. The other comprised non-jihadist groups that are at odds with Nusra.
The insurgent attack in Idlib was led by jihadist groups including Nusra, whereas in Aleppo "there are a range of ideologies ... that mean there is no unity in the military ranks", said an official in the Nour Al-Dein Al-Zenkey Movement, one of the FSA affiliated groups fighting in Aleppo.
The Aleppo rebels are also hindered by fighting with Islamic State, which is vying with them for control of areas north of the city, he said.
"The question of defeating or breaking the regime in Aleppo is difficult, unless the regime collapsed, surrendered and retreated."
(Additional reporting by Laila Bassam, Sylvia Westall and Babak Dehghanpisheh; Editing by Giles Elgood)