Dutch police officers on Sunday in Donetsk, Ukraine. Dmitry Lovetsky/Associated Press
ZUHRES, Ukraine — Just hours after the Malaysian government reached an agreement with Ukrainian separatists on Sunday over access to the crash site of a Malaysian airliner shot down in rebel territory, the Ukrainian military launched an operation to recapture the debris fields, again stalling international efforts to secure the site.
The heavy fighting threatened to torpedo hopes of a breakthrough and cause yet more delays in collecting evidence and retrieving the remaining bodies from the crash. Ukrainian security officials said they needed control over the site to prevent the pro-Russia separatists from destroying clues to the downing of Malaysia Airlines Flight 17.
By Sunday evening, the Ukrainian advance had blocked a road leading from the provincial capital, Donetsk, to the airplane debris northeast of Shakhtyorsk, but it remained unclear whether government troops were in control of all or part of the approximately 14 square miles of debris fields.
Videos posted online appeared to show Ukrainian armored vehicles near the site, and reporters who visited earlier Sunday said insurgents were nowhere to be seen.
The combat spread out along the road in a fluid and chaotic scene, leaving it wholly unclear who controlled what. Fragments of rockets lay on the sunbaked macadam, and columns of black smoke rose along the horizon.
One separatist commander at a checkpoint outside Shakhtyorsk, about 10 miles from the crash site, said the Ukrainians had retaken the area, and a rebel leader, Alexander Borodai, confirmed that government troops were advancing.
“The attempts to clear militia from the crash site irrefutably show Kiev is trying to destroy evidence,” he told reporters in Donetsk. His claim was apparently intended to counter earlier allegations that the rebels had been tampering with evidence to hide their own role in the downing of the plane.
Separatists seemed to be in a state of alarm, driving in convoys of buses and armored vehicles out of Donetsk toward the fighting. They controlled the road as far as the town of Zuhres.
The Malaysian jetliner, a Boeing 777-200, was shot down over eastern Ukraine on July 17 en route from Amsterdam to Kuala Lumpur, Malaysia, killing all 298 people aboard. Ukrainian and American officials say the plane was shot down by a Russian-made surface-to-air missile fired by the rebels. Russia and the rebels have denied any involvement and blame Ukraine.
Ukraine and the United States have said repeatedly that Russia is providing military equipment to the separatists and claim to have evidence that Russia is firing artillery and rockets on Ukrainian military positions.
On Sunday, the Obama administration stepped up its public pressure on Moscow, as the State Department released intelligence images presented as evidence that Russian forces had fired across the border.
The images were said to show charred ground on the Russian side of the border, described as evidence of rocket launches into Ukraine. Another showed artillery pieces of a type found only in the Russian military, pointed toward Ukraine. Other images showed crater impacts inside Ukraine.
It was not possible to independently verify the images. They are from DigitalGlobe, which provides high-resolution satellite images and aerial photos; they were not from American spy satellites or surveillance aircraft. Small groups of foreign police officers and forensic experts have managed to reach the crash site, but efforts to secure it with larger contingents have repeatedly fallen through.
Digital Globe, via U.S. State Department.
Earlier Sunday, the prospects of a more robust foreign presence at the crash site seemed to have improved when the office of Prime Minister Najib Razak of Malaysia announced in an email that he had reached an agreement with Mr. Borodai “to allow a deployment of international police personnel” to enter.
After the announcement, about 30 unarmed Dutch police officers left the eastern Ukrainian city of Kharkiv intending to reach the debris fields. But fighting stopped the officers after they reached Donetsk, said a spokeswoman for the Organization for Security and Cooperation in Europe.
The Dutch police deployment on Sunday, ordered overnight by the Ministry of Security and Justice in The Hague, reversed an earlier decision by the head of a Dutch police mission in Kharkiv. He had intended to delay movement toward the crash site until a vote on Thursday by the Ukrainian Parliament in Kiev that he said would provide a “legal basis” for the deployment of foreign police officers.
The Netherlands, whose citizens accounted for around two-thirds of the crash victims, is leading an international effort to get to the bottom of what happened to Flight 17.
Retrieving the Remains of Flight MH17: Russian Roulette
The area is tactically important for the Ukrainian military, which is trying to close access to Donetsk from the east, lest separatists in the city be resupplied and reinforced from the direction of the Russian border.
Clashes flared in half a dozen towns east of Donetsk on Sunday. There was also fighting to the north, with an artillery strike in the town of Horlivka reportedly killing at least 13 civilians.
The longer the crash site remains unguarded, the smaller the chances of recovering evidence. Responding to growing reports that the wreckage and passenger items had been tampered with, Australia said Sunday that it was sending unarmed police officers to the site to prevent any further meddling. Australia lost dozens of citizens on Flight 17.
“Our objective is to get in, to get cracking and to get out,” Prime Minister Tony Abbott of Australia said at a news conference in Canberra, the capital. Australia had considered allowing some of its officers to carry weapons, but Mr. Abbott said he had decided against that.
“This is a risky mission, no doubt about that,” he said, “but all the professional advice I have is that the safest way to conduct it is unarmed as part of a police-led humanitarian mission.”
Foreign access to the site has been hampered by problems from the start, with heavily armed rebels initially restricting the movements of foreign experts. In Kiev, Andriy Lysenko, a spokesman for Ukraine’s National Security and Defense Council, said the Ukrainian troops intended to “liberate” the crash site to secure evidence.
The Ukrainian government has been loath to see foreign governments negotiate with the separatist leaders based in Donetsk, the capital of a self-declared republic that no foreign state, including Russia, has recognized. Malaysia has been particularly active in reaching out to the rebel leadership. It brokered a deal last week under which the rebels handed over the plane’s data and voice recorders, which they had seized at the crash site.
Andrew E. Kramer reported from Zuhres, and Andrew Higgins from Kharkiv, Ukraine. Chris Buckley contributed reporting from Hong Kong, and Michelle Innis from Sydney, Australia.