Syrian helicopters bombarded a Damascus suburb on Monday and Turkey scrambled warplanes near the border in the north, as the United Nations human-rights chief warned that arms supplies to both the government and rebels were deepening the 16-month conflict.

Fighting has come to the gates of the capital in recent weeks and is also raging throughout the country as the battle to unseat President Bashar al-Assad increasingly takes on the character of an all-out civil war, fuelled by sectarian hatred.

Syrian government forces have launched an assault on Douma, a city on the edge of Damascus where they stormed a rebel stronghold two days ago, leaving bodies rotting in the streets of the nearly abandoned town.

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"The bombardment of Douma continued today using helicopters. Some activists entered the city today and they saw at least seven decaying bodies in the streets under the sun. One man had been executed inside his house," said Mohamed Doumany, an activist who fled the city two days ago and was now nearby.

"There is huge destruction in the city, which is almost empty. Only a few of its people remain inside," he told Reuters by Skype.

The bitterness of the struggle has seen the rebel ranks swollen by defections among troops. Dozens more, including a general, fled into Turkey on Monday, sources in the Free Syrian Army there said. Turkish media said 85 soldiers had deserted.

UN human-rights chief Navi Pillay decried the flow of weapons to both sides. "The ongoing provision of arms to the Syrian government and to its opponents feeds additional violence," she said in the text of remarks made to the Security Council, obtained by Reuters. "Any further militarization of the conflict must be avoided at all costs."

Ms. Pillay did not say where the weapons were coming from, though Russia and Iran are the government's main suppliers. UN diplomats say Qatar and Saudi Arabia have been transferring arms to the increasingly militarized opposition while the United States says it is supplying only "non-lethal" aid to the rebels.

Diplomats from the West and Arab states who oppose Mr. al-Assad met the Syrian leader's allies Russia and China on Saturday in Geneva under the auspices of peace envoy Kofi Annan. However, they made no progress in persuading Moscow and Beijing to sign up to a statement calling for Mr. al-Assad to give up power, leaving the effort to forge an international consensus in tatters.

The failure of diplomacy to have any measurable effect on a conflict that the United Nations says has killed more than 13,000 people is testing the patience of countries in the region, especially Turkey, which reacted with fury 10 days ago when Syria shot down one of its warplanes.

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Turkey said on Monday it had scrambled six F-16 fighters in response to three separate incidents of Syrian helicopters approaching the border. Turkey also scrambled fighters on Saturday and has moved guns and soldiers toward the frontier.

Turkish Foreign Minister Ahmet Davutoglu told Syrian opposition figures gathered in Cairo that their struggle to unseat Mr. al-Assad would end in victory.

"The Assad regime's guns, tanks, weapons have no meaning in the face of the will of the Syrian people. Sooner or later the will of the Syrian people shall reign supreme. And you will lead this process," he said.

Prime Minister Recep Tayyip Erdogan, a former ally of Mr. al-Assad who has turned decisively against him, says Turkish military rules of engagement have been changed and any Syrian forces approaching the border and deemed threatening will be targeted.

The Syrian government tightly controls access, making it difficult to verify accounts of fighting on the ground.

NATO Secretary-General Anders Fogh Rasmussen played down concerns about the military buildup by Turkey, a member of the alliance. Asked whether there was a risk it could lead to a confrontation with Syria, Rasmussen told a news conference in Brussels: "No, on the contrary. I commend Turkey for having shown restraint despite the very tragic aircraft incident."

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"I find it quite normal that Turkey takes necessary steps to protect its population and its territory," he said.

In Cairo, where Syrian oppositions figures gathered, Turkey and anti-Assad Arab states urged the divided opposition to unite and form a credible alternative to the government.

The unity calls were made at the opening of a two-day meeting organised by the Arab League to try to rally Syria's opposition, which has been beset by in-fighting that diplomats say has made it tougher for the world to respond to the crisis. "It is not acceptable to waste this opportunity in any way. The sacrifices of the Syrian people are bigger than us all and more precious than any differences or individual and party interests," Arab League Secretary-General Nabil Elaraby said, addressing the roughly 200 Syrian politicians and activists.

Diplomats and officials at the talks, which are being boycotted by the Free Syrian Army which is leading the armed struggle against Mr. al-Assad's forces, said they did not expect a major agreement to emerge but hoped for some progress.

Anti-Assad activists said there were heavy clashes in Deir Ezzor province near the Iraqi border where villages were under army fire. Rebels destroyed two tanks, they said.

In rural areas near Aleppo south of the Turkish border there were clashes following explosions inside the city overnight. Forested areas near the border were on fire, activists said.

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Syrian artillery pounded the village of Talbiseh near Homs on Monday, targeting an area near the mosque. Video footage posted on YouTube showed a blast hitting the mosque's slender minaret, engulfing it in a cloud of grey smoke and dust.

Other footage showed high explosive rounds slamming into an unseen target behind the mosque every minute.

Security forces were also shelling towns in the province of Deraa, near the Jordan border, activists said.