The rebels vowed to secure the site and allow the recovery of bodies, the Organization for the Security and Co-operation in Europe (OSCE) said.
Malaysia Airlines plane, carrying 298 people, crashed in rebel-held territory on Thursday.
The two sides in Ukraine’s civil conflict have accused each other of shooting the jet down with a missile.
The Boeing 777 was flying from Amsterdam to Kuala Lumpur. It fell between Krasni Luch in Luhansk region and Shakhtarsk in the neighboring region of Donetsk.
Malaysia Airlines said flight MH17 was carrying at least 154 Dutch nationals, 27 Australians, 43 Malaysians (including 15 crew), 12 Indonesians and nine Britons.
Other passengers came from Germany, Belgium, the Philippines and Canada. The dead include world-renowned Dutch researcher Joep Lange who was among a number of passengers en route to an international AIDS conference in Australia.
It is the second disaster suffered by Malaysia Airlines this year. Flight MH370 disappeared en route from Malaysia to China in March and has still not been found.
The separatists pledged to provide assistance in the crash probe after holding a video conference with senior representatives from the OSCE, Ukraine and Russia.
In a statement, the OSCE said the rebels had agreed to “close off the site of the catastrophe and allow local authorities to start preparations for the recovery of bodies”.
They would also provide “safe access” to international investigators and OSCE monitors and co-operate with Ukrainian authorities.
Ukraine has declared the area a no-fly zone, while other airlines have announced they are now setting flight paths to avoid eastern Ukraine.
Describing the disaster as a “tragic day” in a “tragic year”, Malaysian PM Najib Razak earlier said the investigation “must not be hindered in any way”.
Rescue workers said on Friday they had recovered one of the plane’s black box flight recorders after searching through debris spread across several miles.
The Interfax-Ukraine news agency earlier reported that another black box was found by separatist fighters and handed over to Moscow. Observers say the move, if confirmed, is likely to cause international controversy.
US and Ukrainian officials said they believed the plane had been brought down by a missile – a Buk missile system said to have been used by the rebels in Ukraine before.
Ukraine’s President Petro Poroshenko called the disaster an “act of terrorism”.
Ukrainian authorities have released what they say are intercepted phone conversations that proved the plane was shot down by pro-Russian separatists.
The rebels were said to have seized the Buk after overrunning a Ukrainian military base.
However, Ukraine’s Prosecutor General Vitaliy Yarema has cast doubt on this, telling local media: “The military told the president after the passenger plane had been shot down that the terrorists did not possess our Buk missile systems.”
Russian President Vladimir Putin has blamed the Ukraine government for restarting military operations in the area, where it is trying to regain control from pro-Russian rebels.
“The country in whose airspace this happened bears responsibility for it,” he said.
Russia has called for a “thorough and unbiased” investigation, adding that the tragedy also highlighted a need for a swift end to the Ukrainian conflict.
Separatist leader Alexander Borodai also accused the Ukrainian government of downing the airliner.
Ukraine’s defense ministry said there were no air force jets in the area and no surface-to-air systems being used against the rebels.
Ukraine has accused Russia’s military of supplying advanced missiles to the rebels.
Earlier on Thursday, Ukrainian officials blamed the Russian air force for shooting down one of its ground attack jets on Wednesday, and a transport plane on Monday.
The UN Security Council is to hold an emergency meeting on the disaster on Friday morning in New York.
oBudK5Yegk4 ve6SR9_xLhoAt least 158 people have died in Spain's worst flooding disaster in generations. On October…
Google has been fined two undecillion (a two followed by 36 zeroes) roubles by a…
Embarking on a home remodel is an exciting journey, promising enhanced comfort, increased property value,…
The US presidential candidates continued to campaign across key swing states on October 20. Footage…
Elon Musk has said he will give away $1 million a day to a registered…
India and Canada have decided to expel their top envoys along with other diplomats as…