An international team of investigators abandoned efforts to reach the crash site of Malaysia Airlines Flight 17 in eastern Ukraine for a fourth day Wednesday because of heavy fighting, and a Ukrainian official said approaches to the area have been mined.

Military spokesman Andriy Lysenko predicted that sometime “soon,” the Ukrainian army would oust the pro-Russian separatist rebels who currently control the fields and towns where the debris is scattered. But even then, he said, investigators will not be able to visit the site until the mines, which he said have been laid by rebels, can be cleared.

Nearly two weeks after the airliner was shot down by a missile apparently fired from separatist territory, a sense of urgency to get a team of forensics experts to the wreckage, where human remains and plane parts are unguarded, keeps butting up against the dangers of a war zone. The Boeing 777 was carrying 298 passengers and crew en route from Amsterdam to Kuala Lumpur. More than 200 sets of remains have been transferred to the Netherlands for processing and identification, but dozens of others are believed to be unaccounted for.

Deputy Prime Minister Volo­dymyr Groysman told reporters Tuesday in Kiev that “a clear plan has been developed which will allow us to carry out another powerful attempt to enter this area to conduct all activities.”

Groysman stopped short of saying the Ukrainian military was about to overrun the rebels who control the territory around the crash site, but he said he hoped that “in the next few hours, in the next 24 hours, there will be positive news in the fulfillment of this task.”

The latest impasse in the crash investigation developed a day after President Obama announced new sanctions against Russia and called on President Vladi­mir Putin to stop supporting the pro-Russian separatists in eastern Ukraine, pull his troops back from the Ukrainian border and promote a negotiated settlement between the separatists and the Western-backed Ukraine government.

Obama unveiled a new round of U.S. measures that he said would affect “key sectors of the Russian economy.” Among other actions, he expanded a list of Russian banks and defense companies subject to U.S. sanctions.

The European Union earlier Tuesday imposed a wave of tough new sanctions on Russia, including an arms embargo and limits on access to European capital markets for Russian state-owned banks.

Russia reacted sharply Wednesday, professing bafflement at Obama’s accusation that it has not been cooperating with the international investigation of the Malaysian plane crash. U.S. officials have implicated Russia in the tragedy, saying the airliner was shot down with a Russian-supplied surface-to-air missile from separatist-held territory and that the rebels have been armed and trained by Russia.

“Russia presented its existing data from objective monitoring connected to the air crash,” Russian Foreign Ministry spokesman Alexander Lukashevich said in a statement Wednesday. “At the same time, the American side up to this day has not demonstrated any facts, except for unsubstantiated accusations and information from social networks.”

Lukashevich also accused the Dutch officials leading the investigation of delaying an invitation to Russia to participate.

In an interview with Russia’s Interfax news agency, Moscow’s ambassador to the European Union, Vladimir Chizhov, said Europe would suffer more from the new sanctions than Russia.

“The damage on the European economy will be comparable to the damage the E.U. is trying to inflict on the Russian economy, if not greater,” Chizhov said Wednesday in Brussels. He said certain carve-outs in the sanctions package made it obvious that Europe was worried about protecting its own interests.

“They reserve some niches on the tracks that are the most sensitive,” Chizhov said, citing arms contracts in the case of France, financial services for the British and energy for Germany and “many other countries.”

“The European Union so far cannot or does not want to abandon the line of stepping up the sanction pressure, which leads nowhere in itself and at least cannot help de-escalate the situation in Ukraine,” Chizhov said.

In eastern Ukraine, a team of about 50 experts and investigators from the Netherlands and Australia remained in Donetsk as shelling in the city struck an apartment building near their hotel Tuesday.

Last week, monitors from the Organization for Security and Cooperation in Europe said access to the site was quite good.

But this week, the Ukrainian military is in the midst of a major offensive against the rebels, and some of the fiercest fighting has been in the general area where the plane came down in pieces on July 17.

The wreckage and what officials suspect are the remains of dozens of passengers and crew are about 40 miles from Donetsk.

The Russian Foreign Ministry on Tuesday protested the fighting near the crash site, saying that the Ukrainian government was trying to impede the inquiry into the attack.

Ukrainian officials insist that government troops are observing a no-fire zone in a 25-mile perimeter around the crash site.

“The point is that this area is in the control of terrorists,” Lysenko told reporters at a briefing Tuesday. “We can’t guarantee the safety of the experts.”

The United Nations estimates that more than 1,100 people have been killed since fighting began in April.

In Donetsk, heavy fighting continued during the day Tuesday, including in the city center, which is a rebel stronghold where Kiev forces in recent days have been making incursions.

Three people were killed and 15 people were injured in fighting since late Monday night, the city said.

In rebel-controlled Luhansk, city officials said five people died Monday when a retirement home was hit by artillery fire. The mayor’s office in Horlivka, near Donetsk, said 17 people were killed in shelling.

Lysenko said militants were using children as human shields and turning back civilians trying to flee Donetsk and Luhansk by car. Lysenko and other government officials claim the insurgents are posing as Ukrainian soldiers and targeting civilians in an attempt to discredit Ukrainian troops. His claims could not be verified, however.

Birnbaum reported from Moscow. William Branigin in Washington contributed to this report.