United Nations, Oct 26 (IANS): A top UN official 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.

"Some estimate that there are as many as 2,000 armed groups in Syria," Valerie Amos said. The UN under-secretary-general for humanitarian affairs was briefing the UN Security Council on the humanitarian situation in Syria, Xinhua reported.

"Clashes amongst these groups are increasingly common and key humanitarian access routes have been cut off by fierce fighting," the top UN official added.

The UN had a convoy that was ready to go as part of the humanitarian relief efforts in Syria, where a political crisis and subsequent fighting have been going on since March 2011, last week, said Amos.

"But we could not get enough drivers as they fear for their lives," she said.

Amos gave the example to illustrate difficulties the international humanitarian relief workers face while working in the Middle East nation.

She reiterated her disappointment at the lack of progress in efforts to alleviate the suffering in Syria and said: "What we are seeing is a deepening of the crisis, more and more people affected and in particular, I express my concern about the extremely brutal and violent nature of this conflict."

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