An aide to Cameron said the move was intended to help unite the opposition and also was an acknowledgement that a protracted conflict had taken hold in Syria. However, British officials will not be giving military guidance.
"He (Hague) has authorised diplomats to have direct contact with military figures on the ground in order to better understand the actual situation and the relationship between political and armed opposition groups," Cameron's office said in a statement.
"This increased contact with the opposition reflects the reality that these groups are increasingly dictating events in Syria," the statement added.
Cameron will visit a camp for Syrian refugees at the end of a trade and diplomacy trip to the Middle East that has also included the United Arab Emirates and Saudi Arabia, and is expected to announce 14 million pounds ($22.4 million) in aid.
Some 340,000 people have fled Syria and registered as refugees since a March 2011 uprising against Assad turned into an armed rebellion and triggered brutal government crackdowns.
Another 1.2 million people have been displaced within Syria, and 32,000 people have died in the violence.
The effort to improve ties with anti-Assad activists comes ahead of a meeting of Syrian opposition figures on Thursday in Qatar, where they will try to forge a united front in a bid to gain more international support.