Islamic State fighters seized most of a vast camp for Palestinian refugees on the outskirts of Damascus from other insurgents on Wednesday, April 1, in a bid to capture a major foothold close to President Bashar al-Assad's seat of power, Reuters reports.

In a separate advance, rebel groups that have received backing from Assad's Arab and Western foes captured a border crossing with Jordan, the first time Damascus has lost it since the start of the conflict, sources on both sides said.

The gains by groups at opposite ends of the insurgent spectrum show the pressures still facing Assad, who has sought to shore up his control over western Syria including the border zone with Jordan and Israel - the last notable foothold of the mainstream rebels who have largely been eclipsed by jihadists.

They come just days after an alliance of Islamist groups captured the northwestern city of Idlib from the government. The Nusra Front, al Qaeda's wing in Syria and one of the groups involved, announced on Wednesday the city would be run according to sharia law.

The ultra-hardline Islamic State already controls swathes of eastern Syria and Iraq and is being targeted by a U.S.-led campaign of airstrikes.

Until Wednesday's assault on the Yarmouk refugee camp, the group did not have a major presence in the area around the capital, where insurgents have mainly been loyal to other groups.

The Yarmouk refugee camp, which was home to half a million Palestinians before the conflict began in 2011, has been held by anti-Assad insurgents and besieged by government troops since the early days of the war. It is a few kilometers from the heart of Damascus, which is still firmly in government control.