Donetsk, Ukraine (CNN) -- The United States said Thursday it has proof that Russia is firing into Ukraine.
Also Thursday, Ukrainian Prime Minister Arseniy Yatsenyuk and his Cabinet announced their resignation in what Ukrainian political experts said was an expected development. Yatsenyuk and his Cabinet will remain in place as an acting government until elections in October.
The Ukrainian government has been battling pro-Russian rebels in eastern Ukraine. More than 400 people have died in the fighting, the government has said.
"We have new evidence that the Russians intend to deliver heavier and more powerful multiple rocket launchers to the separatist forces in Ukraine and have evidence that Russia is firing artillery from within Russia to attack Ukrainian military positions," said State Department spokeswoman Marie Harf.
Two U.S. officials said earlier that Russian troops have broken up into smaller groups and moved closer to the Ukrainian border.
In the political realm, Yatsenyuk told Parliament that he was announcing his resignation after the blocking of government initiatives and the collapse of the coalition.
"At the moment my government does not have the answer" to numerous questions, including how to keep paying government salaries, military expenses and families of flight victims.
A "collapsed coalition has consequences," he said.
The Prime Minister's announcement comes as more bodies from Malaysia Airlines Flight 17 arrived in the Netherlands. The Boeing 777 was shot down by a suspected surface-to-air missile last week over eastern Ukraine, killing all 298 people on board.
Journalist working for CNN detained in Ukraine
Among other developments Thursday:
-- A Ukrainian journalist working as a freelancer for CNN has been detained by pro-Russian rebels.
Armed men from the self-declared Donetsk People's Republic seized Anton Skiba outside a hotel Tuesday in the rebel-controlled city of Donetsk.
An official with the self-declared Donetsk People's Republic initially accused Skiba of "terrorism" and of posting cash rewards for the killing of rebel fighters on his Facebook page. Later the official dropped the accusation about the Facebook posts and said Skiba was being questioned for having multiple forms of identification with different surnames.
On Wednesday, another high-ranking separatist official told CNN that Skiba admitted to being a "Ukrainian agent."
CNN initially chose not to report on Skiba's detention while making efforts to secure his release.
-- Planes bearing the dead from Flight 17 arrived in Netherlands for a second day.
Aircraft carrying the remains of as many as 74 people touched down Thursday at a military base in Eindhoven. Forty wooden caskets were flown in Wednesday and greeted by somber, moving tributes across the country.
Of the citizens from 10 countries who died on Flight 17, the majority, 193, were Dutch.
'Black boxes' in the UK
It took days for Ukrainian rebels who control the area of the crash site to hand over the bodies and the airliner's black boxes to Malaysian officials.
The voice and flight data recorders are now in Britain for what will be a detailed scouring by international analysts that could take weeks. On Wednesday, investigators found that the cockpit voice recorder was damaged, but its memory was intact. There was no sign of tampering.
But Dutch crash investigators leading the inquiry said Wednesday they still don't have everything they need and haven't been able to visit the crash site under safe conditions to carry out their work.
Australia has sent 50 police officials to London in advance of their joining a possible international deployment to eastern Ukraine to secure the crash site, Prime Minister Tony Abbott said Thursday.
Did rebels have missile system?
Accusations continue to fly over who was responsible for shooting down Flight 17.
One pro-Russian rebel commander further complicated the picture by appearing to acknowledge that rebels had in their possession the type of surface-to-air missile system that U.S. officials say was used to bring down Flight 17.
Since the crash a week ago, rebel leaders have repeatedly denied that they had an SA-11, or Buk, missile system, at their disposal.
But in an interview with the news agency Reuters, Alexander Khodakovsky, the commander of the Vostok Battalion in Donetsk, said he knew about the missile system.
"I knew that a BUK came from Luhansk. At the time I was told that a BUK was coming from Luhansk under the flag of the LNR," Khodakovsky told Reuters, making a reference to the Luhansk People's Republic, the principal rebel group in Luhansk.
Luhansk is one of two rebel provinces in eastern Ukraine. The other, Donetsk, is where Flight 17 came down.
"I found out about it when I found out that this tragedy happened. Most likely it was brought back in order to conceal the evidence of its presence," Khodakovsky said in the Reuters interview, which took place Tuesday and was published Wednesday.
He accused the Ukrainian government of provoking "usage of this kind of weapon against a plane with civilians which was flying by."
But Russian media later reported that Khodakovsky later denied that he confirmed that rebels had the missile system, saying his comments were taken out of context. He told the news outlet RT that he was only discussing theories with Reuters and that he did not have information on such a weapon. Reuters said it is sticking by its story.
CNN's Josh Levs, Michael Pearson, Jethro Mullen, Holly Yan, Nick Paton Walsh, Catherine E. Shoichet, Phil Black, Bharati Naik, Barbara Starr, Brian Todd and Elizabeth Joseph contributed to this report.