The United States is expected to move towards formal recognition of the Syrian National Coalition, formed by disparate opposition groups last month, as the government in waiting.
The UK and France are among those already to have taken the diplomatic step as nations seeking Assad's removal try to forge a united push towards a diplomatic transition.
Mr Hague is keen to reassure allies that fears are unfounded of a post-Assad "vacuum" that could be filled by extremist elements.
After meeting the Syrian National Coalition's president in Brussels on Tuesday, Mr Hague said he had been "encouraged by (his) commitment to include Kurds" in it.
Washington proscribed the Islamist al-Nusra Front, one of the resistance groups, as an al Qaida-backed terrorist group that was trying to hijack the rebellion.
It was among forces that helped take control of a large military base on Tuesday amid opposition advances in the 22-month conflict which has claimed more than 40,000 lives.
The Foreign Secretary will use the meeting to press for an escalation of "practical support" for the rebels in their bid to oust the Damascus government. He will appeal for more funds to ease the humanitarian crisis as the United Nations struggles to deal with more than half a million refugees who have fled the country.
As bitter winter weather approaches, the latest official estimates put the numbers displaced by the conflict within Syria's borders at up to three million.