A top UN official on Friday said that clashes among an estimated 2,000 armed opposition groups in Syria have made it much harder for the international humanitarian relief workers to work in the war-torn country.

The statement came as Valerie Amos, the UN under-secretary- general for humanitarian affairs, was briefing the UN Security Council on the humanitarian situation in Syria, where a political crisis and subsequent fighting have been going on since March 2011.

"Some estimate that there are as many as 2,000 armed groups in Syria," Amos said. "Clashes amongst these groups are increasingly common and key humanitarian access routes have been cut off by fierce fighting."

The United Nations had a convoy that was ready to go as part of the humanitarian relief efforts in Syria last week, said Amos, " but we could not get enough drivers as they fear for their lives." Amos gave the example to illustrate difficulties the international humanitarian relief workers would face while working in the Middle East nation.

While speaking to reporters after her briefing the UN Security Council, Amos reiterated her disappointment at the lack of progress in efforts to alleviate the suffering in Syria, saying " What we are seeing is a deepening of the crisis, more and more people affected and in particular I expressed my concern about the extremely brutal and violent nature of this conflict."

Also on Friday, the UN Children's Programme (UNICEF) and the World Health Organization (WHO) announced that they had joined with other partners in mounting a large-scale immunization effort against polio and other vaccine-preventable diseases in Syria and regions beyond.

On the political front, UN spokesman Martin Nesirky told reporters here that the UN-Arab League joint special representative for Syria, Lakhdar Brahimi, was in Qatar Friday, where he met the Emir to discuss the crisis and preparations for an international conference on Syria scheduled to be held in Geneva, Switzerland.