Police in the self-proclaimed Donetsk People's Republic guard a convoy of investigators headed to the wreckage of Malaysia Airlines Flight 17. The international team abandoned its attempt to reach the site for a second day as fighting raged nearby. (Dmitry Lovetsky/AP)

The Obama administration expects Europe to adopt significant new sanctions against Russia this week, including against key economic sectors that the Europeans have resisted targeting in the past, the White House said Monday.

“In turn and in full coordination with Europe, the United States will implement additional measures itself” amid growing evidence that Russian President Vladi­mir Putin is “doubling down” on his efforts to support separatists battling the government in eastern Ukraine, deputy national security adviser Antony Blinken said.

Blinken’s remarks came as heavy fighting moved closer to the wreckage of Malaysia Airlines Flight 17, in an area under separatist control.

The West has said the airliner was downed with a Russian-supplied missile fired from separatist territory, and investigators from the Netherlands and Australia hope to make it to the site before evidence is destroyed. On Monday, however, the investigators were forced to turn back for a second day after hearing explosions and being warned of heavy combat in the area, where the Ukrainian military claimed significant advances against rebels.

Agreement on the sanctions followed a five-way videoconference among President Obama and his counterparts from Britain, France, Germany and Italy — all of whom separately indicated that they would support additional measures against Russia. European Union ambassadors are scheduled to meet Tuesday in Brussels to consider an arms embargo and sanctions against portions of Russia’s financial and energy sectors.

While agreement from leaders of Europe’s four leading governments was seen as a major step forward, it is unclear whether all 28 E.U. members — which operate on the basis of consensus — will support the new sanctions package, drawn up late last week.

All had agreed to ready the package, to be imposed if there was evidence that Russia was expanding rather than withdrawing its support for the separatists.

On Sunday, the United States published overhead surveillance photographs it said proved that Russia was continuing to supply weapons and to fire artillery from its own territory into Ukraine. Russia has also increased deployment of its forces along the Ukrainian border in possible preparation for a “so-called humanitarian or peacekeeping intervention in Ukraine,” Blinken said.

“The latest information,” a spokesman for British Prime Minister David Cameron said, is that “Russia continues to transfer weapons across the border and to provide practical support to the separatists. Leaders agreed that the international community should therefore impose further costs on Russia, and specifically . . . a strong package of sectoral sanctions as swiftly as possible.”

The Obama administration has said it recognizes that Europe, with far larger economic relations with Russia, has more to lose than the United States. The new European sanctions package, while painful, has been carefully drawn to cause as little disruption as possible. An arms embargo would apply only to new contracts, a senior European diplomat said, allowing at least part of a massive French military shipbuilding contract to proceed.

But the downing of the airliner, and Moscow’s refusal to yield in the face of previous sanctions, have strengthened the U.S. case that Europe must act.

Blinken, who spoke to reporters in the White House briefing room, said Russian support for the separatists has increased as they have lost ground.

Ukrainian forces said Monday that they had captured the separatist stronghold of Saur Mogila, where two Ukrainian planes were shot down last week. A military spokesman characterized the army’s success there as a major victory, since it simultaneously blocks a supply route for rebels from Russia, opens a corridor for Ukraine to resupply its own troops and gives the military control of mountaintop positions used to fire on government forces.

The Ukrainian army is fighting in a circle around the crash site, with the aim of ousting the rebels from the area, military spokesman Andriy Lysenko told reporters Monday in Kiev. “We will force them to leave the site.”

The fighting Monday appeared to be taking place well within a 25-mile zone around the crash site where President Petro Poroshenko vowed troops would not fight or shell, but Lysenko said the Ukrainian military was not conducting operations there.

Chunks of debris from the downed airliner, and possibly human remains, are still spread around the countryside and villages in eastern Ukraine.

The continued difficulties in accessing the site appeared to be frustrating international observers Monday.

“There’s a job to be done,” said Alexander Hug, the deputy head of a monitoring team from the Organization for Security and Cooperation in Europe, according to the Associated Press. “We are sick and tired of being interrupted by gunfights, despite the fact that we have agreed that there should be a cease-fire.”

Lysenko said experts analyzing the flight data recorders from the plane have turned up findings showing that the aircraft experienced a “massive explosion, decompression” consistent with being struck by a missile. Investigators in the Netherlands and Britain, however, have not said anything publicly about the contents of the recorders other than that they have begun to listen to them. If the plane went down because of a missile, not a mechanical failure, the recorders may not yield much useful information.

Also Monday, the top U.N. human rights official said the shoot-down of the plane, with 298 passengers and crew members aboard, “may amount to a war crime.”

Russian Foreign Minister Sergei Lavrov said it would be a grave mistake for the United States and Europe to send military aid to Ukraine, as some U.S. lawmakers have urged.

“Seeing how the Ukrainian authorities have been trying to resolve the so-called problem of the southeast, I think such decisions would directly add oil to the fire and would ignite the belligerent and uncompromising instincts of Ukrainian leaders,” Lavrov told reporters in Moscow.

He called for a U.N.-sponsored inquiry into the fate of the downed Malaysian airliner, and he again rejected U.S. and Ukrainian accusations that Russia is funneling weapons to rebels across the border and shelling Ukrainian military positions from the Russian side.

A top rebel leader, Vladimir Antyufeev, blamed the Ukrainian government Monday for hampering international access to the plane’s debris site, saying the military was attacking rebel positions in the area and making it impossible for international investigators to make the journey from Donetsk, about 40 miles to the west.

“Due to the continuation of the situation, they’re in Donetsk,” Antyufeev told journalists in that city Monday.

In a possible sign of stresses on rebel leadership, Antyufeev also announced that he was becoming the acting head of the Donetsk rebels after Alexander Borodai, another top leader, left for Moscow for consultations with unnamed individuals. Although Borodai, a Russian citizen and former resident of Moscow, has previously traveled to the Russian capital on rebel business, and no handover of power has been made public, it was unclear whether Borodai would be returning.

Morello reported from Kiev. Michael Birnbaum in Moscow contributed to this report.