By Anton Troianovski and Naftali Bendavid

European leaders threatened harsher sanctions against Russia on Sunday in the wake of the downing of Malaysia Airlines Flight MH17, departing from initially muted tones in reacting to the disaster but leaving uncertainty over how quickly the European Union would be able to make good on its threats.

Momentum built on Sunday for the European assets of prominent Russian businessmen and companies to be frozen as early as Tuesday, when EU foreign ministers meet in Brussels.

Anger in EU capitals about the crash, widely believed to have been caused by a missile fired from territory held by pro-Russia rebels in Eastern Ukraine, mounted over the weekend as rebels hindered international efforts to inspect the crash site and recover victims' remains.

U.S. intelligence assessments indicate that Moscow likely provided the pro-Russia separatists in eastern Ukraine with sophisticated antiaircraft systems in recent days, matching evidence put forward by Ukraine and bolstering charges that Russia was the source of the weapon that shot down the airliner, killing 298.

U.S. officials say they suspect that Russia supplied the rebels with multiple SA-11 antiaircraft systems by smuggling them into eastern Ukraine with other military equipment, including tanks.

Further, U.S. officials believe the systems were moved back across the border into Russia following the shoot down of the jetliner, buttressing what Ukraine charges is an attempt by the rebels and their Russian advisers to cover up their involvement in the crash.

Officials who have been pushing Europe to take a harder stance believe the crash marks a watershed. "In my assessment, this is really a turning point," Lithuanian foreign minister Linas Linkevicius said in an interview Sunday. "If anyone had the illusion that this was only happening in Ukraine, that it had nothing to do with our common concerns--now we can see all these victims from the West. European countries are changing their minds and perceptions."

But while support for stiffer sanctions appeared to be growing, EU officials warned that legal and bureaucratic hurdles could make it difficult to implement by Tuesday a major move to punish Moscow for lack of cooperation.

French President François Hollande, after phone calls with U.K. and German leaders, demanded the Kremlin compel pro-Russian separatists in Eastern Ukraine to provide "free and total" access to the Malaysia Airlines crash site.

"If Russiadoes not immediately take the necessary measures, consequences will be decided by the European Union when its foreign-affairs council meets on Tuesday," Mr. Hollande's office said, publicly hinting for the first time since the crash that France would support ratcheting up sanctions.

Italy, a skeptic of sanctions in the past, also issued some of its toughest language yet. "If Russia doesn't cooperate with the investigation" into the circumstances of the plane crash, "we are very much ready to support the sanctions," a spokeswoman for the Italian Foreign Ministry said.

But it remained unclear how hard Europe would be willing to push Russia to alter its overall policy in Ukraine, where western leaders allege Russia has continued to supply separatist rebels with heavy weapons.

German Chancellor Angela Merkel, who has been at the center of Europe's diplomacy, worked the phones on Sunday consulting with European counterparts but made no public statement. Her office declined to comment on statements by Mr. Hollande and U.K. Prime Minister David Cameron indicating the EU's three most powerful leaders agreed new sanctions against Russia were necessary if Moscow didn't change course.

Mr. Cameron expressed his frustration Sunday at reluctance in Europe to confront Russian President Vladimir Putin, and urged a more determined European stance in the wake of the crash.

"For too long, there has been a reluctance on the part of too many European countries to face up to the implications of what is happening in Eastern Ukraine," Mr. Cameron said in an article in London's Sunday Times. "Sitting around the European Council table on Wednesday evening, I saw that reluctance at work again," he said, referring to last week's EU summit.

European diplomats had initially hoped Moscow would respond cooperatively to the crash, and that this could even lead to a broader political reconciliation regarding Ukraine. In the immediate aftermath of Thursday's crash, European leaders largely avoided directly blaming the Kremlin.

But over the weekend EU officials became increasingly convinced that the separatists' denials of responsibility were false.

Russia, meanwhile, stuck firmly to its line that Ukraine created the conditions for the Malaysia Airlines flight to be shot down by pressing its military operation against armed rebels in the east, without indicating who it believes fired the missile that apparently hit the plane.

Russian state television has given plenty of airtime to theories that a Ukrainian fighter shot down the airliner, or that it was a failed attempt to shoot down Mr. Putin's plane, which was in the air over Europe around that time.

Moscow called for an independent, international probe into the crash. In the past, Russia has played down its level of influence on the rebels.

Russia's ambiguous stance, along with reports that bodies were being cavalierly handled and separatist guards on the scene were drunk, has caused fury in European countries where victims came from, including the Netherlands.

A key question is whether anger over the crash propels EU countries to move to "Stage 3" sanctions, aimed at not just individuals but broad sectors of the Russian economy, such as banking or energy. The U.S. has imposed restrictions on exports to Russia that could be used in weaponry, but the EU has yet to take such an action.

The bloc's 28 foreign ministers will almost certainly discuss that question Tuesday, but a final decision would have to be made by EU leaders. Many countries fear retaliation from Moscow should they adopt broader sanctions. It is not yet clear if the anger over the plane crash can overcome those reservations.

European leaders were already frustrated before the crash by what they saw asRussian intransigence on Ukraine. Last Wednesday, they agreed to broaden sanctions to potentially anyone supporting Russian decision makers involved in the Ukraine conflict, and directed EU staff to draw up a list of people and entities to be sanctioned by the end of July.

Officials in several European capitals said the plane crash added a new sense of urgency to drawing up that list, which is also expected to be discussed by EU foreign ministers on Tuesday. But the officials cautioned that the process of drafting the list could take time, given the legal complexity of connecting business entities to events in Ukraine.

Stacy Meichtry in Paris, Deborah Ball in Rome and James Marson in Moscow contributed to this article.

Write to Anton Troianovski at anton.troianovski@wsj.com and Naftali Bendavid at naftali.bendavid@wsj.com

(END) Dow Jones Newswires

July 20, 2014 13:22 ET (17:22 GMT)

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