Paris - The United States is offering Russia a new military pact against the Islamic State (ISIS) and al-Qaeda in Syria, according to a leaked U.S. proposal.If finalised, the arrangement could dramatically alter America's role in the Arab country's five-year civil war.The document published by The Washington Post calls for joint bombing operations, a command-and-control headquarters and other synchronised efforts. US and Russian officials with expertise in intelligence, targeting and air operations will "work together to defeat" the extremist groups, the eight-page paper states.US Secretary of State John Kerry, who was to discuss the proposal in Russia's capital later on Thursday, declined to comment."I'm going to Moscow, meeting with President [Vladimir] Putin tonight," Kerry told reporters in Paris. "We'll have plenty of time to talk about it and I'll give you all a sense of where we are."Dmitry Peskov, Putin's spokesperson, said only that joint US-Russian efforts were key to fighting terrorism in the region.Such a partnership would undercut months of US criticism of Russia's military intervention in Syria. And it would put the US alongside Syrian President Bashar Assad's chief international backer, despite years of American demands for Assad to leave power.Russia would be getting what it has wanted since intervening in Syria in late September: An international alliance of sorts. Washington previously rebuffed Moscow's requests for military co-operation, accusing the Russians of using anti-terrorism objectives as a pretext for protecting Assad's position. The US also says Syria's military and Russia's air force have repeatedly violated truces with moderate rebel groups backed by the US or its allies.No agreement yetUS officials said no agreement with Moscow has yet been reached. Prospects for such a deal are unclear."We're not going to comment on details of documents that have not been approved or agreed to," said a senior State Department official, who wasn't authorised to speak on the matter and demanded anonymity.The proposed, US-Russian "Joint Implementation Group" would be headquartered near Amman, Jordan. At its most basic level, the former Cold War foes would share intelligence and targeting information. But they "should co-ordinate procedures to permit integrated operations", if the US and Russia decide such operations are in their interests.Russia would confine airstrikes to vetted targets and not let Syrian forces bomb "designated areas". Some exceptions apply.The military partnership is part of what US officials are terming a final offer to Moscow. In exchange, the US wants the Russians to pressure Assad into ending a bombing campaign against moderate militant groups and civilian populations, and allowing unfettered aid to besieged, rebel-held areas. Washington also wants Russia's help in forcing Assad to start a political transition that would ultimately end his family's four-decade hold over the country.Russia supports the vague idea of "transition", but has never publicly spoken of Assad having to resign.The proposal would address one of the most persistent problems with efforts to enforce a ceasefire in Syria: the Nusra Front, al-Qaeda's Syria affiliate. The group is engaged in a variety of local alliances with other rebel groups the US and its Arab allies want shielded by the so-called cessation of hostilities. And Nusra's fighters are often embedded with such groups on the battlefield or move between various fighting formations.For that reason, the US has almost entirely avoided bombing Nusra targets in recent months. Russia hasn't hesitated. But in taking out Nusra forces, the US says Russia also has killed hundreds of moderate, anti-Assad fighters and civilians - undermining chances for peaceful diplomacy.Thursday's talks in Moscow are scheduled fewer than three weeks before an August ultimatum by President Barack Obama's administration for diplomatic progress. All signs augur poorly for a breakthrough in a war that has killed as many as a half-million people since 2011, contributed to a global migration crisis and spawned IS' international expansion.Fighting rages near Aleppo, Syria's largest city. Assad has reasserted control over several areas of the country he had once lost. Humanitarian aid deliveries are sporadic and grossly insufficient. And counterterrorism campaigns against the ISIS and al-Qaeda show no end in sight, meaning any peace would only be partial.Two months ago, Kerry said the transition had to start on August 1, or Syria and its backers are "asking for a very different track". But any Plan B has remained undefined beyond vague hints of a military intervention involving Saudi troops. The White House and Pentagon have resisted a greater US role.Wary of working too closely with Russia Much of Washington is wary about working too closely with Russia. The US doesn't want to be seen as entrenching Assad, whom American officials have referred to as a "butcher" and "mass murderer". Russia's bombers also have attacked anti-Assad rebel groups that have received weapons, training and other forms of support from the US and allies such as Saudi Arabia - whose foreign minister Kerry met in Washington earlier this week.And a dissent cable signed by 51 State Department officials last month showed a sizeable part of America's diplomatic establishment believing a US military response against Assad's forces was necessary, given Moscow's increased leverage as a result of its intervention.Opposition to the administration's newest Syria plan is shared by a significant number of officials at the State Department, Pentagon and US intelligence community, according to several American officials.But beyond reaching out to Russia, the administration has few other options right now. Suggestions of US force don't carry much weight, given the various, unfulfilled threats throughout the war - from Obama's declaration five years ago that Assad's days were "numbered" to his vow of a military response if chemical weapons were used, only to back down in 2013.