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AMMAN (Reuters) - At least six people were killed on Sunday when a mortar bomb hit the entrance to a major trade fair being held in the Syrian capital, a war monitor said.

The Syrian Observatory for Human Rights which tracks the conflict did not give further details of the incident which state media has not reported although social media was cited.

Rami Abdulrahman, the head of the Observatory, did not rule out other causes that may have caused the blast. The monitor said the number of casualties was expected to rise with some of the ten people who were injured in critical condition.

The Damascus International Fair being held in the southern outskirts of the capital was opened last week for the first time since the summer of 2011, only months after countrywide unrest erupted.

The exhibition is being attended by regional and foreign businessmen from over 45 countries and authorities have hailed it as a sign that the war-torn country was recovering from the conflict.