An attack on a pro-government television channel's offices near Damascus killed seven staff on Wednesday, state media said, a day after President Bashar al-Assad said Syria was in a ‘state of war.’

A human rights watchdog said the past week had been the bloodiest since the uprising against Assad's rule broke out in March last year and as the exiled opposition called for international action over a week-long bombardment of the eastern city of Deir Ezzor.

UN-Arab League envoy Kofi Annan is pressing for an international conference on the 15-month conflict to go ahead on Saturday despite wrangling between Moscow and Washington over the terms of reference and guest list, his deputy said.

The Syrian delegation stormed out of a UN Human Rights Council meeting after it heard a report that abuses were taking place on a regular basis in a conflict that was taking on an increasingly sectarian dimension.

Live footage broadcast by state television showed extensive damage to the studios of the Al-Ikhbariya satellite channel, with several small fires still burning, in what it described as an unprecedented attack on the pro-government media.

"The terrorist groups stormed the offices of Al-Ikhbariya, planted explosives in the studios and blew them up along with the equipment," Information Minister Omran al-Zohbi told the television in a live interview.

"They carried out the worst massacre against the media, executing journalists and security staff," Zohbi said.

State media said the dead comprised three journalists and four security guards.

"This didn't come out of nowhere," he added, pointing to European Union sanctions imposed on the pro-government media.

Al-Ikhbariya remained on the air despite Wednesday's assault.

With the uprising now in its 16th month, Assad told his cabinet on Tuesday that Syria was in a ‘real situation of war’ and ordered ministers to crush the anti-regime revolt.

"When one is in a state of war, all our policies and capabilities must be used to secure victory," he said, according to the official SANA news agency.

More than 15,800 people have now been killed in the uprising, of whom 4,681 lost their lives since the UN-backed ceasefire brokered by Annan was supposed to take effect on April 12, the Britain-based Syrian Observatory for Human Rights said.

"The last week was the bloodiest week of the Syrian Revolution," Observatory director Rami Abdel Rahman said, adding that 916 people were killed from June 20 through 26.

The latest report to the UN Human Rights Council from the Independent Commission of Inquiry on Syria found that the violence in Syria was increasingly taking on a sectarian basis amid mounting enmity between the Sunni Muslim and Assad's minority Alawite community.

"Where previously victims were targeted on the basis of their being pro- or anti-government, the CoI has recorded a growing number of incidents where victims appear to have been targeted because of their religious affiliation," it found.

The findings triggered a walkout by the Syrian delegation.

"We will not participate in this flagrantly political meeting," said Syrian Ambassador Faisal Khabbaz Hamoui.

The opposition Syrian National Council called for urgent action by the international community to help civilians trapped by a regime bombardment of rebel neighbourhoods of Deir Ezzor.

"The Syrian regime has been brutally shelling the city of Deir Ezzor, and nearby villages and towns for the past week, using heavy artillery, tanks and helicopters," the opposition bloc said.

It said the bombardment had left "hundreds killed or wounded and a large segment of the residential neighborhoods destroyed, leaving thousands of citizens displaced."

Annan's deputy, Jean-Marie Guehenno, told the UN Human Rights Council that the peace envoy wanted a mooted international meeting on the conflict to go ahead in Geneva on Saturday.

He said the meeting was planned at ministerial level and it was essential for states with influence to agree among themselves "in order to effectively support a Syrian-led transition process."

"The envoy is using his best efforts to facilitate a common position on the proposed outcomes of the Action Group," he said.

"Those outcomes will have to be meaningful and concrete in order to help alleviate the crisis."

His comments came after Washington rejected a Russian proposal for Iran to take part in the planned meeting and demanded that a transition from Assad's rule be put squarely on the agenda.

State Department spokeswoman Victoria Nuland told reporters in Washington that the US did not think Iran could make a useful contribution given its support for the Damascus regime.

State Department officials have also said Secretary of State Hillary Clinton will not attend the meeting unless all parties first agree on the need for political change in Syria.