Tony Blair has told the US inquiry into future policy options that resolving the Israel-Palestinian conflict is crucial to the future of Iraq.
The PM's appearance was via video link to the US
He told the Iraq Study Group the biggest factor in getting support from moderate Muslim countries was progress on Israel and Palestine.
Speaking by video link he told the group a plan for Iraq and the region as a whole was needed, his spokesman said.
The ISG is expected to report its findings to President Bush in weeks.
The prime minister's spokesman added later that Mr Blair thought there was a "realistic" chance of progress on the Israel-Palestine conflict and he intended to visit the region before the end of the year.
'Axis of Evil'
Resolving the conflict would stop moderate Muslims being stirred up and increase pressure on Iran and Syria to co-operate, Mr Blair told the group.
He also said Syria and Iran had to be given the "strategic choice" between being part of the solution or "isolation".
The ISG, headed by former US Secretary of State James Baker, is thought to be considering encouraging Iran and Syria to take a role in bringing stability to Iraq.
But President Bush has indicated that there would be no softening on policy towards Iran and Syria - countries he once described as being part of an "Axis of Evil".
In his speech to the Lord Mayor's Banquet on Monday night, Mr Blair said partnership was possible if Iran helped the Middle East peace process, stopped supporting terrorism in Lebanon or Iraq and abided by its international obligations.
There has been no official response from the Iranian government, but Sadegh Ziba Kalam, professor of politics at Tehran University, said Mr Blair's "bullying" language had disappointed many Iranians.
"Mr Blair was actually threatening the Iranian leader," he said.
"That wasn't at all the gesture and atmosphere of the talks that one expects when one is trying to seek a conciliatory atmosphere."
President Bush met the ISG on Monday
Shadow foreign secretary William Hague told the BBC it was sensible to invite Iran and Syria to talks "on the right terms" but a wider group of countries in the region needed to work together.
But he added: "It would be a mistake, I think, to imagine that in the next few weeks these countries are going to turn around and say: 'Oh yes, we will help you out in Iraq'.
He added later that the prime minister "must give a frank assessment of the situation in Iraq to Parliament when he speaks tomorrow".
Lib Dem leader Sir Menzies Campbell agreed Iran and Syria had a role to play.
But he said it "is galling that the prime minister will outline his ideas regarding a strategy change to the Baker Commission, when the government has refused Parliament the same opportunity".