Iyad Allawi said on Monday that he held the discussion recently in Damascus.
He did not say when the meeting took place, but Allawi's term as interim prime minister ended in late April.
He added that during his talks with al-Assad, he touched on the possible reactivation of a tripartite committee bringing together officials from Syria, Iraq and the United States to deal with outstanding problems.
The committee was set up last year and held only one meeting in Damascus.
The US and Iraq accuse Syria of turning a blind eye to the use of its territory by fighters crossing into Iraq to join the anti-US uprising.
They have in the past accused the Syrians of harbouring senior members of ousted president Saddam Hussein's government who allegedly have played an active role in financing and organising the armed campaign.
Allawi said the proposed buffer zone would be manned by UN observers.
US and Iraqi officials say Syria is
letting fighters cross into Iraq
He did not say whether al-Assad agreed to such a proposal.
However, he later briefed two senior US State Department officials in Amman, Jordan, about it.
Their initial response was "positive", he added.
Allawi said he also wrote to Iraqi President Jalal Talabani and Prime Minister Ibrahim al-Jaafari informing them of the proposal.
"I hope this will be taken forward," he said. "There will not be 100% control over the border, but at least there will be substantial control. So, we are moving one of the problems or causes that's helping terrorism."
"While I was talking to the [Syrian] president, I said 'you know what is happening in Iraq may happen in Syria'," Allawi said, alluding to the possibility that the violence in Iraq may eventually spill over into Syria.
"It will, of course, give Iraq a lot of leverage in fighting terrorism and closing or sealing the border," he said.
"While I was talking
to the [Syrian] president, I said 'you know what is happening in Iraq may happen in Syria'"
ex-interim Iraqi prime minister
Allawi is a secular Shia politician who had spent three decades in exile abroad before he returned to Iraq after Saddam's ouster in 2003.
His party won 40 of parliament's 275 seats in last January's landmark elections.
On Monday, he said he was working to forge alliances with various political groups, especially among Iraq's Kurds and moderate Muslim groups, to contest the next election, scheduled for December.