Mottaki told reporters that Iran would continue to provide money to Hamas, but he did not give any figures.
Iran is known to have given $120 million to Hamas since it took office following its victory in the Palestinian legislative elections.
Mahmoud Ahmadinejad, the Iranian president has urged visiting Meshaal on Tuesday to keep fighting Israel, state television said.
The United States and Israel accuse Iran of "interference" in Iraq, through backing Shia fighters, and of sponsoring terrorism, including Hamas, which the US still labels as a terrorist organisation and Lebanon's Shia-led group Hezbollah. Tehran denies the charges.
"The [Palestinian] government should use its brave and pious forces to continue resistance against the Zionist regime [Israel]", Ahmadinejad told Mashaal at the start of his two-day visit to Iran.
Iran refuses to recognise Israel and Ahmadinejad has called the Israeli state a "tumour" which must be "wiped off the map".
Iran's support for the Palestinians has grown more vocal since Ahmadinejad came to power in August 2005.
The major aid donors to the Palestinian Authority - the United States, the European Union and Canada - cut off aid because Hamas has refused to renounce violence and recognise Israel and the previous agreements signed between Israel and the Palestinians.
At Tuesday's press conference, Meshaal was asked if Hamas had now recognised Israel.
The Hamas leader did not answer directly, but said: "The Palestinian government insists on June 4, 1967 borders [for a Palestinian state], full Palestinian sovereignty with Jerusalem as its capital."
He was referring to the international borders before the 1967 Arab-Israeli war, which broke out on June 5 that year, in which Israeli troops captured the West Bank, east Jerusalem and the Gaza Strip.
Last month, Meshaal and Mahmoud Abbas, the Palestinian president, signed a Saudi-brokered agreement in Mecca under which they would form a coalition government and Hamas promised to "respect" the previous agreements.
The United States and others are waiting for the coalition to be formed before declaring whether the Mecca agreement warrants a resumption of aid.
Mottaki said his government backed the Mecca accord.
"Iran supports this initiative, and it also supports any step toward achieving greater unity among Palestinians," he said.