By Lesley Wroughton and Stephanie Nebehay
VIENTIANE/GENEVA (Reuters) - The United States said on Tuesday it hoped to announce in early August details of planned military cooperation and intelligence sharing with Russia on Syria, and a United Nations envoy said he would also aim to resume peace talks next month.
U.S. Secretary of State John Kerry said Washington and Moscow, which support opposing sides in Syria's five-year-old conflict, had made progress in recent days towards working more closely together.
The proposals would have the two powers share intelligence to coordinate air strikes against the al Qaeda-affiliated Nusra Front and prohibit the Syrian air force from attacking rebel groups labeled as moderate.
Efforts to bridge the divide between the United States and Russia and bring Syrian government and opposition forces back to negotiations come after pro-government forces have effectively put rebel-held districts of Aleppo under siege.
Concern is growing for at least 250,000 people who have been trapped in rebel-controlled eastern Aleppo since early July, and the U.N aid chief asked on Monday for weekly 48-hour pauses in fighting to allow food and aid to be delivered.
Syrian state television said on Tuesday the army had sent text messages to residents and fighters in eastern Aleppo, saying it will grant safe passage to whoever wants to leave and asking militia to put down their weapons.
The Syrian Observatory for Human Rights monitoring group said 25 people, including three women and eight children, were killed in the last 24 hours in the Mashhad quarter of rebel-held Aleppo, when it was hit by barrel bombs thrown from helicopters.
In Geneva, U.N. Special Envoy for Syria Staffan de Mistura said he aimed to convene a new round of peace talks toward the end of August, quietly scrapping a previous Aug. 1 deadline to reach agreement on a framework for a political transition.
"Our aim, let me say very clearly, is to proceed with a third round of intra-Syrian talks towards the end of August," De Mistura told reporters after meeting U.S. Syria envoy Michael Ratney and Russian Deputy Foreign Minister Gennady Gatilov.
De Mistura said he strongly hoped Kerry and Russian Foreign Minister Sergei Lavrov would make concrete and visible progress because that would improve the situation on the ground and the environment for the peace talks, although such progress was not a precondition for talks.
The U.S. State Department said in a statement after the Geneva meeting that Washington and Moscow had urged the United Nations to prepare a proposal for political transition - based on Security Council resolutions and input from Syrian parties - which would serve as the starting point for future talks.
The statement said Washington had emphasized the need to restore "compliance with the terms of the cessation of hostilities - particularly in Aleppo city - as well as the need to improve humanitarian access, as positive progress in these areas would significantly improve the prospects for successful talks".
Kerry has defended the proposal despite deep scepticism among top American military and intelligence officials, including Secretary of Defense Ash Carter and chairman of the Joint Chiefs of Staff Joseph Dunford, over working with Russia.
"My hope is that somewhere in early August we would be in a position to stand up in front of you and tell you what we're able to do with the hopes it can make a difference to lives of people in Syria and to the course of the war," Kerry said after meeting Lavrov.
During the discussions, he and Lavrov outlined the next stage of implementing the plan, including a series of technical-level meetings to address concerns by the U.S. military and intelligence officers.
Kerry's State Department and White House allies say the plan is the best chance to limit the fighting that is driving thousands of Syrian civilians - with some trained Islamic State fighters among them - into exile in Europe, and preventing humanitarian aid from reaching tens of thousands more.
A senior Western diplomat said the lack of transparency of the U.S.-Russia talks was frustrating and - with what the diplomat said was increased targeting of civilians and hospitals on the ground - it was hard to foresee any deal.
"The Americans are risking a lot for a deal that is as unlikely to be honored as previous engagements the Russians have made," the diplomat said.
Another diplomat said it was unlikely De Mistura would meet his new target of resuming talks in August. "In reality it means there will be nothing in August, it means September," the diplomat said.
"The window of opportunity is extremely limited after that ... Military cooperation has become a pre-requisite,(peace talks) won’t advance without it".
Syria's government said on Sunday it was ready for further peace talks with the opposition and that it was intent on a political solution to the conflict.
Basma Kodmani, a member of the main Syrian opposition High Negotiations Committee, said the HNC was "open to going back to Geneva to discuss options" but added that there must be a reduction in the current level of violence.
"The situation is absolutely horrendous at this moment," she told Reuters. "What Mr De Mistura is probably hoping is that the agreement between Russia and the U.S. will result in a halt to the regime's raids, and Russian air raids as well."
Russia's military intervention in support of President Bashar al-Assad last year helped to turn the tide of the war after the government had lost ground to rebel fighters in the west of the country.
On Sunday Syrian government air strikes put four hospitals in Aleppo province out of action, while several people in the government-controlled ancient quarter of Damascus were killed when a mortar bomb hit a restaurant.
International humanitarian groups have condemned the tightening siege on rebel-held parts of Aleppo city.
"Food there is expected to run out over the next few weeks," a joint statement from international aid groups including Oxfam and Mercy Corps said on Tuesday. "A food warehouse was also targeted with almost 10,000 food parcels destroyed, while fuel ... is dangerously low," the statement said.
The United Nations said a 48-truck convoy of international aid organizations was heading to the besieged areas of Talbiseh in northern Homs province, carrying food aid for 40,000 people.
(Additional reporting by Tom Miles in Geneva and Dominic Evans and Lisa Barrington in Beirut; Editing by David Stamp and Bill Rigby)