Iraq's Prime Minister Nouri al-Maliki has warned that arming either side in Syria will lead to a "proxy war".
He was speaking at the opening of an Arab League summit which is discussing a joint plan with the UN to end a year of violence in Syria.
Syrian President Bashar al-Assad has agreed to the plan and will spare no effort to make it succeed, Syrian state news agency Sana reported.
The summit is the first such meeting in Baghdad for at least two decades.
President Assad's remarks were contained in a message to the world's emerging powers - Brazil, Russia, India, China and South Africa - who are meeting in Delhi on Thursday.
He said he would work to enforce the peace plan but "terrorism" must stop as well, Sana reported.
The Syrian government blames the year-old anti-government uprising on foreign-backed terrorists and armed criminal gangs.
The UN-Arab League plan, brokered by envoy Kofi Annan, would see a UN-monitored end to fighting, troops pulled out of opposition areas and access for humanitarian services.
Syria first agreed to the initiative on Tuesday but violence has continued.
A number of explosions were heard in central Baghdad as the summit was getting under way.
Two of the blasts occurred near the Iranian embassy, eyewitnesses said. There are unconfirmed reports that an explosion near the city's secure Green Zone was an IED (improvised explosive device).
Fewer than half the Arab League's 22 heads of state turned up for the summit, reflecting their suspicion of Iraq's government and its close ties to non-Arab Iran. Iran is also Syria's closest ally in the Middle East.
"Based on our experience in Iraq, the option to arm either side of the conflict will lead to a regional and international proxy war in Syria," Mr Maliki said.
The Emir of Kuwait is attending the summit - the first visit by a Kuwaiti leader to Baghdad since Kuwait was invaded by Iraq under Saddam Hussein in August 1990.
Qatar and Saudi Arabia, the two Arab states most fiercely opposed to the Syrian government, have only sent envoys.
The summit is being held in such tight security at the city's former Republican Palace that the venue was not initially disclosed to journalists.
While expectations are not high for the talks, the fact that they are being held in the Iraqi capital at all can be seen as a sign of progress for Iraq, says the BBC's Wyre Davies, in Baghdad.
UN Secretary General Ban Ki-moon urged Syria "to put commitments into immediate effect".
"The world is waiting for commitments to be translated into action. The key here is implementation: there is no time to waste," Mr Ban told the summit.
Mr Ban is due to meet key leaders to discuss how the UN can work with the Arab League to put the Annan plan into action.
'Keep pressure on'
The UK will give £500,000 ($795,000) to Syrian opposition groups, Foreign Secretary William Hague is to announce later on Thursday.
The money will be used for "practical non-lethal support" and to document human rights abuses by the Syrian government, Mr Hague will say in a speech in London.
Both Qatar and Saudi Arabia have voiced their support for arming opposition forces in Syria, and some analysts believe they are already funnelling weapons to the rebels.
Washington has urged countries to maintain pressure on the government of President Bashar al-Assad.
The US state department said it had "not seen the promises that Assad made implemented".
"It's incumbent on all of us to keep the pressure on Assad to meet the commitment that he's made, and that's our intention over the next few days," a spokesperson said.
Syria has said it will not address any initiative from the Arab League, from which it was suspended last year.
The opposition in Syria is sceptical about the terms of Mr Annan's plan, with some saying Mr Assad is merely stalling for time in order to continue his crackdown.
"We are not sure if it's political manoeuvring or a sincere act," said Louay Safi, a member of the opposition Syrian National Council.
"We have no trust in the current regime... We have to see that they have stopped killing civilians."
The UN says more than 9,000 people have been killed during the year-long Syrian revolt.
Opposition activists say at least 40 people have been killed since government troops overran the opposition-held town of Saraqeb in the north-west at the weekend.
Corpses littered the streets as homes were burned to the ground and shops pillaged and looted, they said, in reports which could not be verified independently.
Much of Baghdad has been brought to a standstill for the summit, which is costing an estimated $500m (£314m) to stage.
The Iraqi government is hoping to re-establish itself in the Arab fold after years of violence and sectarian conflict, Wyre Davies reports from Baghdad.
Little progress is expected either on the Syrian front or on wider tensions between Shia and Sunni factions in the region, he notes.
But if the summit, which is expected to last for barely a few hours, passes off without incident and if there are no insurgent attacks elsewhere in the country, it will be seen as a resounding success, our correspondent adds.