BEIRUT — Gunmen stormed a pro-government Syrian TV channel headquarters on Wednesday, bombing buildings and shooting dead three employees, state media said, in one of the boldest attacks yet on a state symbol.

President Bashar Assad declared late on Tuesday that his country was “at war”. US intelligence officials said the Syrian government was “holding fairly firm” and digging in for a long struggle against rebel forces who are getting stronger.

The dawn attack on Ikhbariya television’s offices, 20km south of the capital, as well as overnight fighting on the outskirts of Damascus showed the 16-month-old violence now rapidly encroaching on the city.

Ikhbariya resumed broadcasting shortly after the attack, which killed three journalists and four security guards, displaying bullet holes in its two-storey concrete building and pools of blood on the floor. One building was almost entirely destroyed.

“I heard a small explosion then a huge explosion and gunmen ran in. They ransacked the offices and entirely destroyed the newsroom,” an employee who works at the offices in the town of Drousha told state media at the scene.

Syrian media are tightly regulated by the ministry of information. Although Ikhbariya is privately owned, opponents of Assad say it is a government mouthpiece.

After Tuesday’s unprecedented fighting around Damascus, violence appeared to ease off around the capital following the attack on the television complex.

During the pro-democracy revolt against the Assad family’s four-decade rule, Ikhbariya has been pushing to counter what it says is a campaign of misinformation by Western and Arab satellite channels on the uprising that began in March 2011.

“We live in a real state of war from all angles,” Assad told a Cabinet he appointed on Tuesday, in a speech broadcast on state television. “When we are in a war, all policies and all sides and all sectors need to be directed at winning this war.”

The declaration marks a change of rhetoric from Assad, who had long dismissed the uprising against him as the work of scattered militants in “terrorist gangs” funded from abroad.

International mediator Kofi Annan said he had convened a ministerial-level meeting on Syria in Geneva on Saturday with the aim of seeking an end to the violence and agreeing on principles for a “Syrian-led political transition”.

In a statement, the joint United Nations-Arab League envoy said he had invited foreign ministers from the five major powers, Britain, France, China, Russia and the US, as well as Turkey, the EU, Iraq, Kuwait and Qatar.

Last week’s shooting down of a Turkish warplane by Syrian air defences has focused attention on a volatile situation on Turkey’s southeastern border with Syria.

“We will not refrain from teaching a lesson to anyone trying to test Turkey’s greatness,” Turkish Prime Minister Recep Tayyip Erdogan said on Wednesday, referring to the incident near the countries’ maritime borders.

Turkey’s land border territories, hosting over 33,000 refugees and units of the rebel Free Syria Army, are quickly becoming a potential flashpoint.

Erdogan said on Tuesday that Syrian military elements approaching the border and posing a threat would be deemed a military target.

UN investigators said on Wednesday Syrian government forces had committed human rights violations, including executions, across the country “on an alarming scale” during military operations in the past three months.

The report by the UN Human Rights Council, issued in Geneva, also listed killings and kidnappings by armed opposition groups trying to topple President Assad.

“The situation on the ground is dangerously and quickly deteriorating,” the report said.

Syria’s ambassador dismissed the accusations and threatened to end cooperation with international agencies.

The UN accuses Syrian forces of killing more than 10,000 people during the conflict, which began with a popular uprising and has built up into an armed insurgency.

A White House spokespersonn said of the attack on the pro-government television station: “We condemn all acts of violence, including those targeting pro-regime elements.”

Despite some military defections, mainly from low to mid-level ranks, Assad’s inner circle remains cohesive and the war is still likely to be a drawn-out struggle, senior US intelligence officials said.

“Our overall assessment... would be that we are still seeing the military regime forces fairly cohesive, they’ve learned some lessons over the last year and a half about how to deal with this kind of insurgency,” an official said.

The insurgency is also getting stronger, he added.

“Both sides seem to be girding for a long struggle. Our sense is that the regime still believes it can ultimately prevail or at least appears determined to try to prevail and the opposition at the same time seems to be preparing for a long fight.”