Syrian opposition forces have killed several top officials in President Bashar Assad's regime, rebel leaders reported on Sunday. Among the dead, according to the rebels, were Assad's brother-in-law Assef Shawkat and Defense Minister Dawoud Rajiha.

According to an al-Jazeera report, Free Syrian Army special forces also killed former Defense Minister Hasan Turkmani in an overnight operation near Damascus.

The report could not yet be confirmed, with a rebel spokesperson saying proof would be revealed in the coming hours. Syrian state television denied the report.

According to State Syrian TV, Turkmani and another name mentioned in as a casualty of the alleged overnight operation, Interior Minister Mohammad al-Shaar called in themselves to deny the reports, denouncing al-Jazeera for "spreading lies."

Meanwhile, heavy fighting erupted Sunday between Syrian troops and rebels on the outskirts of the capital Damascus, according to the London-based Syrian Observatory for Human Rights.

The clashes occurred at a security checkpoint in the suburban area of Kafr Soussa, while several blasts were also heard in the area of Douma near Damascus, reported the Observatory. The explosions were caused by shelling attacks mounted by Syrian troops against the area, it added.

On Saturday, a car bomb killed nine people at a Syrian military post in the eastern city of Deir al-Zor, an attack the government said was the latest proof that an uprising against Assad was a foreign plot.

The official SANA news agency said the blast had been the work of a suicide bomber and had also wounded about 100 people, including guards, at what it called military installations.

International pressure and a UN-backed peace plan have failed to quell Syria's turmoil. French President Francois Hollande said on Saturday that the peace plan still had international backing, but Washington sounded a more aggressive note saying Assad had to leave power.

Syrian state television broadcast footage of smoke rising over Deir al-Zor, pools of blood amid rubble, the damaged facades of buildings and twisted, smoking vehicles.

Opposition activists said the target was an intelligence base.

"It seems like a well-planned attack. The explosion hit the least guarded rear gate of the Military Intelligence complex ... where the operatives keep their cars," said one activist in Deir al-Zor.

State television called the blast part of a campaign funded by Saudi Arabia and Qatar to topple Assad.

The Sunni-led Gulf powers have called for military help for the Free Syrian Army, a loosely organised force of defecting soldiers and protesters formed in response to Assad's crackdown on what began 14 months ago as a peaceful uprising.

Syrian television said UN staff, who are supposed to be monitoring an internationally brokered ceasefire, had inspected the site.

The UN-Arab League peace plan drawn up by Kofi Annan aims to mark a political path out of the violence in Syria.

UN peacekeeping chief Herve Ladsous, in Damascus for consultations with ceasefire observers, told reporters on Saturday: "This is a process. We have reached certain elements of our objectives. We are not there yet."

Hollande said leaders of the Group of Eight countries, meeting at Camp David, had agreed to continue supporting Annan's Syrian peace efforts. "I insisted that all the participants support Kofi Anan's mission so that observers can provide protection of the Syrian people from their leaders," Hollande said.

But the White House said Syria's violence would not end without a political changeover, adding that external monitors and a ceasefire would not be sufficient to address the problem. "We believe change has to include (Syrian President) Bashar al-Assad leaving power," White House aide Ben Rhodes told reporters during the G8 summit.

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