Ukrainian Prime Minister Arseniy Yatsenyuk announced his resignation Thursday, creating new uncertainty in his nation at a crucial moment in its military offensive against pro-Russian rebels in the east.

The move was sure to distract Ukrainian politicians even as leaders from around the world push for unfettered access to the wreckage of Malaysia Airlines Flight 17, which was shot down deep inside rebel-held territory. A week after the attack on the Boeing 777-200, investigators still have not been able to examine the site in a systematic manner, partly because of heavy fighting nearby.

As fighting continued in eastern Ukraine, the Obama administration said that it had new intelligence information that Russia is preparing to deliver heavier,
more powerful ground-to-ground
multiple-rocket launchers to separatist forces, and that Russian forces on their own side of the border are firing artillery at Ukrainian military positions.

Dutch military aircraft continued flying the remains of victims of Malaysian Airlines Flight 17 from Ukraine to the Netherlands on Thursday. The Dutch and Australian governments were discussing the formation of an international protection force of military and police from countries with victims of the crash, to protect investigators who are still waiting for full access to the separatist-controlled crash site.

Yatsenyuk’s surprise resignation came after two major parties said they were withdrawing from the governing coalition. President Petro Poroshenko welcomed the coalition’s collapse, saying it bows to Ukrainian society’s desire for “a complete reload of state power.” Poroshenko later said he hoped the “entire” cabinet, presumably including Yatsenyuk, would stay on.

Deputy Prime Minister Volodymyr Groysman was appointed acting prime minister following Yatsenyuk’s resignation. Groysman, 36, has been minister of regional development, and head of a commission to investigate the cause of the downing of Malaysia Airlines Flight 17.

The political rearrangement seemed intended to pave the way for elections this fall, two years early. Poroshenko pledged wide government and electoral reforms when sworn into office last month, but many members of parliament are a holdover from the era of ousted president Viktor Yanukovych and are considered resistant to reforms and the loss of influence.

Nevertheless, Yatsenyuk’s resignation threw the government into disarray at a critical juncture. Finance Minister Oleksandr Shlapak warned parliament Thursday that the military was swiftly running out of money to pay for its offensive in the east, where troops are seeking to regain control of rebel-held territory around Donetsk and Luhansk.

“As of Aug. 1, we’ll have nothing to pay the military,” Shlapak said, according to the Interfax-Ukraine news agency, as he urged parliament to increase tax revenue.

The immediate trigger for Yatsenyuk’s resignation was the decision by the Svoboda and Udar parties earlier Thursday to pull out of the coalition government, which took over five months ago after Yanukovych was driven out of office by sustained protests. Party leaders said their intention was to force early elections.

But the collapse of the coalition, Yatsenyuk said, means that parliament would be politically hobbled as it tries to pass crucial laws on matters such as the military budget and uncertain energy supplies.

“Who wants to go to elections and simultaneously vote for unpopular laws?” he said in announcing his resignation. “Putting narrow political interests above the future of the nation is impermissible. It is a moral and ethical crime.”

Yegor Firsov, a lawmaker from the Udar party, said that it was prepared to support government initiatives and that he was surprised that Yatsenyuk attributed his resignation to the threat of government paralysis.

“Now, it’s a kind of a vacuum,” Firsov said.

But Yatsenyuk’s resignation does not take effect immediately, and some say it is of little practical consequence.

“The government will continue to fulfill its duty before the new parliament will be elected,” said Volodymyr Fesenko, director of the Center of Applied Political Studies in Kiev, the capital.

Others said the move weakens the government’s ability to make forceful decisions.

“It’s not a welcome development,” said John Herbst, director of the Atlantic Council’s Eurasia Center and a former U.S. ambassador to Ukraine. “There is a lot to be concerned about. Government unity is important for dealing with the current security dangers, but this is something for Ukrainians to work out.”

In the combat zones in the east, military officials said they are working to restore infrastructure and order in the towns they have liberated from the rebels and are finding a disturbing degree of destruction.

Andriy Lysenko, a spokesman for the Ukrainian National Security and Defense Council, said schoolyards have been mined, water supplies disrupted and jails emptied of prisoners — sowing fear in the local population.

There was intense fighting in and around Donetsk on Thursday, with explosions and gunfire audible from various parts of the city.

The fiercest clashes appeared to be occurring near the airport. Early in the morning, a column of five armored rebel vehicles could be seen speeding by a hotel where many journalists are staying. Some of the vehicles bore World War II markings and appeared to have been reconditioned.

At the Pentagon, spokesman Col. Steven H. Warren said that “we now know that the Russians have been firing artillery from Russia — Russian troops on Russian soil firing Russian artillery into Ukraine . . . they’ve been doing it for several days.” The artillery is located “close to the border,” Warren said. U.S. officials have previously alleged movement of Russian multiple-rocket launchers in and out of Ukraine, and “we have new evidence that the Russians intend to deliver heavier and more powerful” launchers, State Department spokeswoman Marie Harf said.

Meanwhile, Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, said that the Russian government, probably for the first time since 1939, “has made the conscious decision to use its military force inside of another sovereign nation to achieve its objectives.”

“I think it does change the situation,” he said.

Speaking at a national security conference in Colorado, Dempsey said that he had not spoken with his Russian counterpart in two months but that lines of communication remained open between the two countries’ militaries. He said he believes that senior Russian military officials “are probably reluctant participants” in Russian operations against Ukraine that he described as “proximate coercion and subversion.”

“My fear,” Dempsey said, “is that Putin may actually light a fire that he loses control of.”

Birnbaum reported from Donetsk. Karen DeYoung in Washington, Greg Miller in Aspen, Colo., Karoun Demirjian in Moscow and Alex Ryabchyn in Kiev contributed to this report.