Fighting in Syria between regime troops and rebels are raging in different parts of the country, especially in the south, where opposition fighters are battling to seize control of the border crossing with Jordan.

Opposition activists told Al Jazeera on Wednesday that the fighters were now "in control of most of Ramtha post" in the southern province of Deraa.

The clashes came as the UN inspectors returned to Syria to investigate the use of chemical weapons.

Convoys of inspectors arrived at a Damascus hotel on Wednesday, a week after they confirmed that sarin gas was used in an attack August 21 in the suburbs of the Syrian capital.

The US and its allies accuse President Bashar al-Assad's government of carrying out the attack, a claim the government had repeatedly denied.

The team of investigators is expected to examine the alleged use of chemical weapons up to 14 times in Syria's 30-month conflict.

On the ground, meanwhile, fierce fighting erupted in the strategic northern town of Areeha, forcing locals to seek cover in nearby caves.

Rebels are battling for control of the town, which sits along a key supply route for Assad's forces.

In the northern province of Aleppo, rebels said they seized control of the Madajen district, another key supply route for the regime that is located near the airport road.

Activists in the northeastern province of Raqqa reported the death of five people in air raids by government fighter jets on the city of Tabqa.

Fighter jets also dropped shells near a bakery rebel-held Raqqa city, activists reported, causing several casualties and fire outbreak in the area.

Activist in the Damascus suburb of Ghouta witnessed heavy artillery shelling by regime forces on Wednesday.

The town has been under blockade for several months and medical supplies there are running desperately low.

Opposition revolt

Reports of Wednesday's fighting came as 13 rebel groups released a statement saying they rejected the authority of any foreign-based opposition groups, including the internationally recognized National Coalition.

The groups include members of the Free Syrian Army (FSA), nominally the military arm of the National Coalition, but also groups such as the al-Qaeda-affiliated Jabhat al-Nusra.

Critics of the National Coalition, which is based in Istanbul, Turkey, say it is not transparent with its funding and political functioning.

They say it is out of touch with people in Syria, where more than 100,000 have been killed and large chunks of territory have been destroyed by combat and shelling.

Some Jabhat al-Nusra fighters and supporters who spoke to Reuters news agency said their groups had been courting ideologically moderate rebels as they watched their relationship sour with the National Coalition.

Saudi Arabia, which had taken over as the main foreign player and financier of the National Coalition, is said to have been trying to sideline Islamist rebels and bolster secular forces.

"Our brigades are tired of the National Coalition's strategy. It is acting on the exterior and ignoring the interior," one Nusra supporter said.

"And a lot of the moderate Islamists were shoved aside when Qatar's role was overtaken by Saudi Arabia. The Saudis started aggressively supporting the secular groups."

Against this backdrop of continued fighting and political tensions, the World Food Programme (WFP) has given warning that, with winter ahead, the humanitarian situation in Syria is likely to get worse.

Speaking to Al Jazeera in New York on Wednesday, Ertharin Cousin, WFP's executive director, said: "Things are continuing to escalate and simply get worse. We are feeding three million people inside Syria today.

"We are increasing that number to four million by the end of October, because there are actually 5.8 million people who are in need of assistance inside Syria."

PHOTO CAPTION

A United Nations vehicle is seen at the Lebanon-Syria border as UN inspectors return to Syria on September 25, 2013.

Aljazeera