* Hollande speaks after states agree aid to rebels
* Says Islamist fighters give "pretext" to Assad
* Calls on Iran's new president to "be useful"
Syrian rebels need to wrest back control of territory held by Islamist militants whose involvement in the conflict gives Bashar al-Assad a pretext for more violence, French President Francois Hollande said on Sunday.
Radical Islamist groups such as the al Qaeda-linked Nusra Front have joined in fighting against the Syrian president's forces in the conflict which has killed more than 90,000 people and displaced millions.
"The opposition needs to win back control of these areas ... they have fallen into the hands of extremists," Hollande told a news conference in the Qatari capital Doha.
"If it seems that extremist groups are present and tomorrow they could be the beneficiaries of a chaotic situation, it will be Bashar al-Assad who will seize on this pretext to continue the massacre."
The French president was in Doha for a meeting of 11 Western and Arab countries, known as the "Friends of Syria", over the two-year-old Syrian conflict.
The countries agreed on Saturday to give urgent military support to the rebels, channelled through the Western-backed Supreme Military Council, a move that Washington and its European allies hope will prevent weapons falling into the hands of Islamist radicals.
Hollande said the countries still needed to work out how to supply arms to the Syrian opposition and that deliveries were conditional on the rebels organising politically and militarily.
"We cannot imagine delivering weapons to groups which could use them to the detriment of interests of a democratic Syria or eventually against us," he said.
The aim, he said, was to assert military pressure on Assad, because doing nothing would benefit "Assad on the one hand and the most radical elements on the other ... We refuse that."
He also called on Iran's new President Hassan Rohani to use his influence to help the Syrian situation.
"Elections have taken place in Iran, (there is) a new president. It's up to him to show he can maybe be useful, that he can also exert pressure on Bashar al-Assad to find a solution."
The United Nations said on Sunday it had so far raised 33 percent of the $5 billion of humanitarian aid it was seeking to help the Syrian people.
"The United Nations is trying its best to help the Syrian refugees to help the Syrian refugees but a political solution is the way out of this crisis," Panos Mounties, regional coordinator of the U.N. refugee agency, said after a meeting with the Arab League in Cairo.