AMMAN (Reuters) - The Syrian army and its allies pressed ahead on Friday with a two-week-long offensive to seize a strategic valley where a key spring provides supplies to four million people in the capital, Damascus, residents and rebels said.

Aerial bombing and shelling from the army as well as Hezbollah fighters stationed in the mountains that overlook the valley on the northwestern edge of the capital had intensified in the last forty-eight hours, they said.

Scores of jets pounded the area around the Ain al-Fija springs and the villages of Baseimah, Kafr Zayt and al Husseineh, which form part of a cluster of ten villages controlled by rebels in the valley that lies at the northwestern edge of the capital.

The Syrian army, aided by Hezbollah, the Iranian-backed Lebanese Shi’ite group, has so far been unsuccessful in making any significant advance in the valley since they launched the drive to capture the strategic area and accused rebels of polluting the springs with diesel.

The rebel fighters there rejected a government offer to leave the area for the rebel-held province of Idlib in northern Syria. Similar deals have led rebels to yield swathes of territory, including Aleppo.

Area residents say the rugged nature of the terrain gives the rebels, drawn from both moderate and Islamist factions, a natural advantage to pin down any advancing troops.

“They cannot advance easily; even if the rebels are outnumbered they can easily strike any advancing troops from the three main entrances into the encircled valley,” said Abu Mohammad al Qalamoni, a rebel fighter in contact with comrades inside the valley.

CEASEFIRE STRESSED

The military offensive has strained a ceasefire agreement brokered by Russia and Turkey aimed at bringing about Syrian peace talks in Kazakhstan.

The opposition has warned that unless the Syrian army halts its attacks it would consider any truce “null and void”. They have also suspended any discussion on participating in the forthcoming peace talks unless Russia puts pressure on the government and its Tehran-backed allies to abort the offensive.

Wadi Barada lies on a road from Damascus to the Lebanese border that is a key supply line for Hezbollah, which is heavily involved in fighting alongside the Syrian army.

The rupture of water supplies from the springs has caused severe shortages after the pumping station of Ain al Fija that supplied around 70 percent of the capital’s water needs was damaged.

Prices of bottled water and trucked water supplied by private traders to residential homes has tripled, residents of the sprawling capital say, with a black market now thriving.

The government of Syrian President Bashar al Assad has also brought in supplies from other provinces by tanker to cover some of the shortfall in the capital and pumped extra water from underground wells.

The army says it is fighting radical Islamists in the area, a claim denied by local fighters. A military media unit run by Hezbollah said on Thursday at least 11 al Qaeda-linked fighters were either killed or wounded when they were targeted by rockets, but those figures could not be independently confirmed.

The civilian population in the valley is estimated by the United Nations to number around 45,000, but civic groups say the total is double that with their plight worsening daily under heavy shelling and shortages of food and medicine.

Dozens of homes have been hit by the bombing campaign.

Only 1,200 families have so far left to a government-run shelter in the nearby town of Rawda, the U.N. said.

“We hope in a few days water will return back to the capital after the army takes back Ain al Fija. The army is advancing and ... we expect good news,” Alaa Munir Ibrahim, a governor in the Damascus suburbs, told state media.

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