United Nations: In the wake of Syrian President Bashar Al-Assad's denial over ordering violent crackdown on protesters, UN Chief Ban Ki-Moon has said he "cannot believe" Assad is not responsible for around 4,000 civilian deaths during the past nine months.

In a recent interview with a news channel, Assad had said he was not responsible for his security forces' crackdown on the protesters and that the United Nations was not a credible institution.

Ban responded by saying that the "United Nations has been addressing all these challenges, violations of human rights issues, which have been happening in Syria and elsewhere on a very impartial, fair, objective and credible basis.

"I cannot believe that he (Assad) has not killed more than 4,000 people. All the credible information is that more than 4,000 people have been killed by the government forces. The High Commissioner for Human Rights has made it already clear through all the various sources, very credible sources," Ban said in Kenya in response to questions.

Assad had also dismissed the UN's estimate of the number of protesters killed and said the global body should send "concrete evidence" to support allegations that Syria has committed war crimes, which he called a "distortion of reality".

UN High Commissioner for Human Rights Navi Pillay has said that in addition to the civilian casualties, "I acknowledge that almost 1,000 of President Assad's security forces have also been killed in this conflict and this is why I am alerting the world that, as you have more and more defectors from the security forces, this may well develop into a fully-fledged civil war."

Speaking on the occasion of Human Rights Day, Pillay said there were first-hand eyewitness accounts and a great deal of information received from people both inside and outside the country that gross human rights violations have taken place in Syria.

"It was not good enough for the president to simply brush aside the assertions and say that the United Nations was not a credible enough source," she said.

Gaining access to the country in order to better assess what was going on was a priority, she emphasised, noting that communities were under siege and people did not dare to step outside their homes for fear of being shot.

Pillay is scheduled to brief the Security Council in a closed-door meeting on Syria next Monday.

The report of the independent international commission of inquiry into the nine-month crackdown in Syria released last week had said that security and military forces committed crimes against humanity, including acts of killings, torture, rape and imprisonment of civilians.