* Army says makes advances in eastern Damascus
* 170 killed on Friday as war rages despite chemical deal
* Weapons experts investigating more chemical allegations
By Dominic Evans
BEIRUT, Sept 28 (Reuters) - Rebel fighters including foreign Islamists took control of a military post on Syria's southern border with Jordan on Saturday after four days of heavy fighting with President Bashar al-Assad's forces, activists said.
The former customs post on the edge of the city of Deraa, where Syria's uprising against Assad erupted in 2011, was seized by several groups including the al Qaeda-affiliated Nusra Front and the Islamist Ahrar al-Sham, according to the reports.
A video from the site showed a masked fighter claiming victory in the name of the two movements, and a captured tank from which a black Islamist flag was fluttering.
Rebels also seized a fuel depot north of Damascus, the Syrian Observatory for Human Right said, while state media said Assad's forces made gains in rebel areas of east Damascus.
The fighting across Syria highlights the relentless nature of its civil war, as battles with conventional weapons rage on regardless of steps to destroy Syria's chemical arms - the result of a rare outbreak of international consensus on Syria.
An Aug. 21 sarin gas attack, which prompted global condemnation and led the United States and Russia to bridge some of their deep divisions over the Syrian conflict, killed hundreds of people in rebel-held areas of the capital.
But similar numbers die every week in air strikes and artillery fire by Assad's forces and by guns, rockets, mortars and tanks deployed by both sides.
The British-based Observatory, an anti-Assad group which monitors violence through security and medical sources inside Syria, said 170 people were killed on Friday, including soldiers, rebels and civilians.
Authorities restrict media operations in Syria, making it difficult to confirm reports from activists or official sources.
Syria's state news agency SANA said the army made major advances in the eastern Damascus districts of Jobar, Qaboun and Zamalka, where they found a network of rebel tunnels, used by the rebels to store weapons.
It quoted a local army commander saying one of the rebel tunnels was 150 metres long. In some of the tunnels the army found armour-piercing explosives weighing 30-40 kg, as well as French-made anti-tank rocket launchers and parts of anti-aircraft guns, the report said.
The three eastern districts where the army reported making advances are on the edge of the Ghouta district which was hit by chemical weapons last month.
United Nations inspectors said in a report two weeks ago that sarin was used in the attack. Western powers said the report showed Assad's forces were responsible for the attack, an accusation which the president and his Russian allies have denied, pointing the blame at rebels.
The inspectors returned to Syria on Tuesday to continue investigations into allegations of chemical and biological weapons use, including three previously unreported cases around Damascus in the days following the Aug 21 attack.
A car carrying members of the team was seen leaving their Damascus hotel on Saturday morning but there was no indication of where they were heading. They are due to complete their work in Syria on Monday and present a full report by late October.
The day after that team completes its mission, experts from the international watchdog the Organisation for the Prohibition of Chemical Weapons (OPCW) will begin inspecting Syria's stockpile of chemical weapons, under the terms of a deal struck this month which averted U.S. military action.
(Editing by Andrew Heavens)
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