Iran has offered to help finance the cash-strapped Palestinian Authority governed by the Hamas militant group.
Mr Larijani questioned Washington's commitment to democracy
The offer was announced by senior security official Ali Larijani after a meeting with Hamas politburo chief Khaled Meshaal, state radio reported.
The decision follows moves by the US and Israel to isolate a Hamas-led government with financial penalties.
Hamas has also held initial talks with Fatah, which lost January's election, to discuss forming a coalition.
On Tuesday, Palestinian President Mahmoud Abbas formally asked Hamas to form a government.
The new administration will be headed by one of the group's leaders in Gaza, Ismail Haniya.
Fatah had initially ruled out joining its Islamist rival in government, but the two agreed to try to find common ground for a governing partnership.
"There is an agreement in principle and the intention is there, but we must await the programme," the head of Fatah's parliamentary faction, Azzam al-Ahmed, said after the talks in Gaza City.
Mr Ahmed told Reuters that Fatah would not join Hamas unless the government adopted President Abbas's vision of negotiating peace with Israel.
But Hamas, though it has been observing an informal truce, refuses to recognise Israel and has ruled out negotiations.
Hamas' refusal led to international condemnation and the withdrawal of US aid to the Palestinian Authority last week.
The Israeli government also froze the transfer of millions of dollars in funds to the authority - a move which the UN has called unhelpful and premature.
But on Wednesday, Iran said it would help make up the PA's funding shortfall.
"Khaled Meshaal's request for assistance to enable the [Palestinian] Authority to overcome the existing problems is noted by Iran and we shall definitely help them financially," Mr Larijani told reporters.
Mr Larijani criticised the US government's decision to withdraw aid and questioned its commitment to democracy.
"You know very well that Hamas is a genuine popular movement which has always pursued the objective of recovering the rights of the oppressed Palestinian people; but unfortunately the Americans have never paid any attention to this matter," he said.
"The US decision to stop financial aid shows that they are not seeking to promote democracy in the region, contrary to their claims on the Middle East [road-map] proposal."
On Monday, Iran's Supreme Leader, Ayatollah Ali Khamenei expressed support for Hamas and called on other Muslim nations to provide financial aid.